Leaves are 2 to 6 inches (5-14 cm) long and attached close to the stem [14]. Flower spikes vary in length from > 40 inches (1 m) to only a few inches, and only 2 to 3 inches (5.1-7.6 cm) of the spike typically display open flowers at any given time [9,73]. Fruits are capsules 2-3 mm in length [56]. Seeds measure approximately 400 x 200 microns, and weigh approximately 1.8 x 10-6 ounces (50 µg) per seed, which is comparatively quite small among North American temperate wetland plants [116,129].
Seedlings quickly develop a thick, hardened taproot [111]. Mature plants subjected to persistent flooding respond by forming aerenchymous (containing large intercellular air spaces) tissue, permitting oxygen flow to submerged roots [118].
The preceding description provides characteristics of purple loosestrife that may be relevant to fire ecology and is not meant to be used for identification. Keys for identifying purple loosestrife are available in various floras (e.g. [57,71]). Photos and descriptions of purple loosestrife are also available online from Minnesota Sea Grant. Check with the native plant society or cooperative extension service in your area for more information.
Considered native to Eurasia [125], purple loosestrife has a widespread circumpolar distribution throughout the northern hemisphere, except in extremely cold and arctic regions [111,129]. Although the precise origin of purple loosestrife colonization in North America is unknown, it was well established by the 1830s within coastal wetlands along the New England seaboard, having likely been introduced via ship ballast soil. Further introductions are thought to have occurred intentionally by early American horticulturalists. Initial spread of purple loosestrife into the interior of eastern North America occurred primarily via routes of maritime commerce, such as canals, rivers and the Great Lakes. Spread into the arid West appears to be closely related to development of irrigation systems within that region [129].
The following biogeographic classification systems are presented as a guide to demonstrate where purple loosestrife could potentially be found based on reported occurrence and on biological tolerance to factors likely to limit its distribution. For instance, because purple loosestrife does not tolerate salt water, classifications describing a variety of salt marsh habitats are excluded from these lists. Additionally, many of these classifications are named for predominantly upland habitats that nevertheless contain sometimes-substantial wetland areas where purple loosestrife could potentially occur. Precise distribution information is lacking because of gaps in the understanding of biological and ecological characteristics of non-native species and because introduced species may still be expanding their habitable range. Therefore these lists are speculative and may not be complete.
Fire adaptations: Purple loosestrife is an herbaceous perennial, with growing points that overwinter on the root crown about 0.8 inches (2 cm) below the soil surface (see Botanical and Ecological Characteristics) [129]. FIRE REGIMES: Because purple loosestrife is distributed across many habitats in North America (see Distribution and Occurrence), FIRE REGIMES associated with the species vary. Recurrence and behavior of fire in areas where purple loosestrife occurs is likely to be closely tied to particular local FIRE REGIMES, and cannot be easily summarized over broad spatial scales. Given purple loosestrife's moisture requirements, it is unlikely to occur in areas experiencing frequently recurring fire. Similar to many areas that experience fire infrequently, occurrence of fire in areas where purple loosestrife is found is likely to driven by drought. However, information describing interactions between purple loosestrife and fire are lacking, and information linking purple loosestrife to specific North American FIRE REGIMES is nonexistent.
Given the dearth of information on fire and purple loosestrife and our relatively poor understanding of how purple loosestrife generally affects plant community dynamics where it occurs, any description of interactions between particular FIRE REGIMES and purple loosestrife is speculative. Where purple loosestrife displaces native vegetation dependent upon recurring fire for maintenance of a seral stage, persistent stands of invasive purple loosestrife may alter FIRE REGIMES if purple loosestrife burns less frequently or less readily than the native vegetation it displaces. For example, sedge meadow communities along the St. Lawrence River in southern Quebec where purple loosestrife is sometimes found, are historically maintained by dormant season fire recurring every 1 to 3 years. If invading purple loosestrife reduces fire frequency or severity at these sites, these communities are likely to succeed to woody species such as willow (Salix spp.) or maple (Acer spp.) [8].
The following table lists fire return intervals for communities or ecosystems throughout North America where purple loosestrife may occur. This list is not intended as a description of purple loosestrife distribution, but rather as a guide to FIRE REGIMES in areas where purple loosestrife could potentially be found. (For more specific distributional information see Distribution and Occurrence). While this list mainly describes upland habitats, purple loosestrife is generally associated with wetland or riparian habitats within these communities or ecosystems. As such, this list is meant as a guideline to illustrate historic FIRE REGIMES and is not to be interpreted as a strict description of FIRE REGIMES for purple loosestrife. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) silver fir-Douglas-fir Abies amabilis-Pseudotsuga menziesii var. menziesii > 200 grand fir Abies grandis 35-200 [5] maple-beech-birch Acer-Fagus-Betula > 1000 silver maple-American elm Acer saccharinum-Ulmus americana sugar maple Acer saccharum > 1000 sugar maple-basswood Acer saccharum-Tilia americana > 1000 [135] California chaparral Adenostoma and/or Arctostaphylos spp. 95] bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium 69,95] Nebraska sandhills prairie Andropogon gerardii var. paucipilus-Schizachyrium scoparium bluestem-Sacahuista prairie Andropogon littoralis-Spartina spartinae sagebrush steppe Artemisia tridentata/Pseudoroegneria spicata 20-70 basin big sagebrush Artemisia tridentata var. tridentata 12-43 [108] mountain big sagebrush Artemisia tridentata var. vaseyana 15-40 [6,25,85] Wyoming big sagebrush Artemisia tridentata var. wyomingensis 10-70 (40**) [133,143] coastal sagebrush Artemisia californica saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus desert grasslands Bouteloua eriopoda and/or Pleuraphis mutica 5-100 plains grasslands Bouteloua spp. blue grama-needle-and-thread grass-western wheatgrass Bouteloua gracilis-Hesperostipa comata-Pascopyrum smithii blue grama-buffalo grass Bouteloua gracilis-Buchloe dactyloides cheatgrass Bromus tectorum California montane chaparral Ceanothus and/or Arctostaphylos spp. 50-100 [95] sugarberry-America elm-green ash Celtis laevigata-Ulmus americana-Fraxinus pennsylvanica 135] paloverde-cactus shrub Cercidium microphyllum/Opuntia spp. 95] curlleaf mountain-mahogany* Cercocarpus ledifolius 13-1000 [7,109] mountain-mahogany-Gambel oak scrub Cercocarpus ledifolius-Quercus gambelii 95] Atlantic white-cedar Chamaecyparis thyoides 35 to > 200 [135] blackbrush Coleogyne ramosissima northern cordgrass prairie Distichlis spicata-Spartina spp. 1-3 [95] beech-sugar maple Fagus spp.-Acer saccharum > 1000 [135] California steppe Festuca-Danthonia spp. 95] black ash Fraxinus nigra 135] juniper-oak savanna Juniperus ashei-Quercus virginiana Ashe juniper Juniperus ashei western juniper Juniperus occidentalis 20-70 Rocky Mountain juniper Juniperus scopulorum cedar glades Juniperus virginiana 3-7 tamarack Larix laricina 35-200 [95] western larch Larix occidentalis 25-100 [5] creosotebush Larrea tridentata Ceniza shrub Larrea tridentata-Leucophyllum frutescens-Prosopis glandulosa 95] yellow-poplar Liriodendron tulipifera 135] melaleuca Melaleuca quinquenervia 88] wheatgrass plains grasslands Pascopyrum smithii 95] Great Lakes spruce-fir Picea-Abies spp. 35 to > 200 northeastern spruce-fir Picea-Abies spp. 35-200 [33] southeastern spruce-fir Picea-Abies spp. 35 to > 200 [135] Engelmann spruce-subalpine fir Picea engelmannii-Abies lasiocarpa 35 to > 200 [5] black spruce Picea mariana 35-200 conifer bog* Picea mariana-Larix laricina 35-200 [33] blue spruce* Picea pungens 35-200 [5] red spruce* P. rubens 35-200 [33] pine-cypress forest Pinus-Cupressus spp. 5] pinyon-juniper Pinus-Juniperus spp. 95] whitebark pine* Pinus albicaulis 50-200 [5] jack pine Pinus banksiana 33] Mexican pinyon Pinus cembroides 20-70 [86,126] Rocky Mountain lodgepole pine* Pinus contorta var. latifolia 25-300+ [4,5,107] Sierra lodgepole pine* Pinus contorta var. murrayana 35-200 [5] shortleaf pine Pinus echinata 2-15 shortleaf pine-oak Pinus echinata-Quercus spp. 135] Colorado pinyon Pinus edulis 10-49 [95] slash pine Pinus elliottii 3-8 slash pine-hardwood Pinus elliottii-variable sand pine Pinus elliottii var. elliottii 25-45 [135] South Florida slash pine Pinus elliottii var. densa 1-5 [88,135] Jeffrey pine Pinus jeffreyi 5-30 western white pine* Pinus monticola 50-200 Pacific ponderosa pine* Pinus ponderosa var. ponderosa 1-47 interior ponderosa pine* Pinus ponderosa var. scopulorum 2-10 [5] Table Mountain pine Pinus pungens 135] red pine (Great Lakes region) Pinus resinosa 10-200 (10**) [33,37] red-white-jack pine* Pinus resinosa-P. strobus-P. banksiana 10-300 [33,54] pitch pine Pinus rigida 6-25 [24,55] pocosin Pinus serotina 3-8 pond pine Pinus serotina 3-8 eastern white pine Pinus strobus 35-200 eastern white pine-eastern hemlock Pinus strobus-Tsuga canadensis 35-200 eastern white pine-northern red oak-red maple Pinus strobus-Quercus rubra-Acer rubrum 35-200 loblolly pine Pinus taeda 3-8 loblolly-shortleaf pine Pinus taeda-P. echinata 10 to Virginia pine Pinus virginiana 10 to Virginia pine-oak Pinus virginiana-Quercus spp. 10 to sycamore-sweetgum-American elm Platanus occidentalis-Liquidambar styraciflua-Ulmus americana 135] galleta-threeawn shrubsteppe Pleuraphis jamesii-Aristida purpurea eastern cottonwood Populus deltoides 95] aspen-birch Populus tremuloides-Betula papyrifera 35-200 [33,135] quaking aspen (west of the Great Plains) Populus tremuloides 7-120 [5,45,84] mesquite Prosopis glandulosa mesquite-buffalo grass Prosopis glandulosa-Buchloe dactyloides Texas savanna Prosopis glandulosa var. glandulosa 95] black cherry-sugar maple Prunus serotina-Acer saccharum > 1000 [135] mountain grasslands Pseudoroegneria spicata 3-40 (10**) [4,5] Rocky Mountain Douglas-fir* Pseudotsuga menziesii var. glauca 25-100 [5] coastal Douglas-fir* Pseudotsuga menziesii var. menziesii 40-240 [5,87,106] California mixed evergreen Pseudotsuga menziesii var. m.-Lithocarpus densiflorus-Arbutus menziesii California oakwoods Quercus spp. 5] oak-hickory Quercus-Carya spp. 135] oak-juniper woodland (Southwest) Quercus-Juniperus spp. 95] northeastern oak-pine Quercus-Pinus spp. 10 to 135] oak-gum-cypress Quercus-Nyssa-spp.-Taxodium distichum 35 to > 200 [88] southeastern oak-pine Quercus-Pinus spp. 135] coast live oak Quercus agrifolia 5] white oak-black oak-northern red oak Quercus alba-Q. velutina-Q. rubra 135] canyon live oak Quercus chrysolepis blue oak-foothills pine Quercus douglasii-Pinus sabiana 5] northern pin oak Quercus ellipsoidalis 135] Oregon white oak Quercus garryana 5] bear oak Quercus ilicifolia 135] California black oak Quercus kelloggii 5-30 [95] bur oak Quercus macrocarpa 135] oak savanna Quercus macrocarpa/Andropogon gerardii-Schizachyrium scoparium 2-14 [95,135] shinnery Quercus mohriana 95] chestnut oak Q. prinus 3-8 northern red oak Quercus rubra 10 to post oak-blackjack oak Quercus stellata-Q. marilandica black oak Quercus velutina live oak Quercus virginiana 10 to135] interior live oak Quercus wislizenii 5] cabbage palmetto-slash pine Sabal palmetto-Pinus elliottii 88,135] blackland prairie Schizachyrium scoparium-Nassella leucotricha Fayette prairie Schizachyrium scoparium-Buchloe dactyloides little bluestem-grama prairie Schizachyrium scoparium-Bouteloua spp. tule marshes Scirpus and/or Typha spp. 95] redwood Sequoia sempervirens 5-200 [5,36,124] southern cordgrass prairie Spartina alterniflora 1-3 [95] baldcypress Taxodium distichum var. distichum 100 to > 300 pondcypress Taxodium distichum var. nutans 88] western redcedar-western hemlock Thuja plicata-Tsuga heterophylla > 200 [5] eastern hemlock-yellow birch Tsuga canadensis-Betula alleghaniensis > 200 [135] western hemlock-Sitka spruce Tsuga heterophylla-Picea sitchensis > 200 mountain hemlock* Tsuga mertensiana 35 to > 200 [5] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. 33,135] *fire return interval varies widely; trends in variation are noted in the species summaryNorthern limits of purple loosestrife distribution may be strongly influenced by low growing season temperature. Under controlled conditions, growth was severely restricted at 46.4 degrees Fahrenheit (8 °C) compared with more "characteristic" growth at 64.4 degrees Fahrenheit (18°C) [113].
Purple loosestrife is found on both calcareous and acidic soils [111,113,129] and tolerates low-nutrient soils [111,117,141]. Typically found in open areas, purple loosestrife will tolerate some shade, but growth, reproduction and survival may be substantially reduced under shaded conditions [110,118].
Several characteristics of wetland or riparian habitats have been identified that may be predictive of invasibility by purple loosestrife. Assuming dispersal is largely via floating propagules, isolated wetland basins may be less susceptible to purple loosestrife colonization than areas with interconnected waterways. Additionally, narrow streams with steep gradients are probably less susceptible, because they are frequently scoured and contain fewer areas of slack water, while slower, broader flows are more likely to contain habitat suitable for colonization. Riparian areas that are mostly shaded are also less susceptible because purple loosestrife seedlings require relatively high light levels. Finally, the presence of one or more commonly associated taxa, such as cattails (Typha spp.), reed canarygrass (Phalaris arundinacea), sedges (Carex) spp., and rushes (Juncus spp.) may indicate a habitat that is highly susceptible to invasion by purple loosestrife [129].
Because purple loosestrife has demonstrated strong competitive abilities where it has invaded North American wetland communities, there is concern that it may diminish native plant diversity. For instance, competition with purple loosestrife has been suggested as a contributing factor in the decline of the rare Long's bulrush (Scirpus longii) in Massachusetts [28]. However, studies published to date have failed to demonstrate a deleterious effect of purple loosestrife on native plant diversity. Treberg and Husband [130] examined the association between purple loosestrife abundance and vascular plant richness along the Bar River in Ontario. Purple loosestrife had been present in this area for at least 12 years and there was a wide range in established plant densities. They found no significant (P<0.05) difference in mean species richness associated with the presence or percent cover of purple loosestrife, and no plant species was significantly (P<0.05) more likely to be found in the absence of purple loosestrife than in its presence. Anderson [1] showed no significant (P<0.05) correlation between total species richness and either percent cover, genet density or median age of purple loosestrife, even in plots containing 18-20 year old purple loosestrife plants. He suggested areas with apparent purple loosestrife monocultures perhaps had low species richness to begin with, and species richness more likely resulted from habitat heterogeneity rather than the presence of innately competitive species. More research is needed in this area.
Purple loosestrife colonization has been purported to have detrimental effects on birds, based on: a) creation of unsuitable nesting habitat and b) low food potential of purple loosestrife relative to vegetation it displaces. However, published studies and observations indicate impacts on birds are not yet clear. Marsh wrens prefer cattails to purple loosestrife for nesting [101,142]. There is speculation that invasion of riparian areas in Nebraska may have adverse effects on important night-roosting habitat for migratory sandhill cranes. Purple loosestrife invasion is predicted to have detrimental effects on nesting habitat of black terns and canvasbacks in the north-central United States, but this has not been tested [129]. Whitt et al. [142] found purple loosestrife-dominated habitats had significantly (P=0.003) higher bird densities but significantly (P=0.03) fewer bird species than other habitats. These higher densities were mainly due to increases in populations of a single species, the swamp sparrow.
Purple loosestrife colonization can substantially reduce or eliminate open water in small marsh areas, potentially reducing its usefulness for waterfowl. In areas with substantial seed banks, mudflats that are commonly used as feeding areas by shorebirds are impacted by rapid, dense colonization by purple loosestrife seedlings. Decline in the extent of open water habitats from increased emergent purple loosestrife can retard access to aquatic prey items such as fish and aquatic invertebrates. Important aquatic food plants for wildlife such as pondweeds (Potamogeton spp.) are inhibited under the shade of emergent purple loosestrife [102]. Invading purple loosestrife in coastal British Columbia's Fraser River estuary may have negative effects on detrital food chains [44].
Thompson and others [129] have illustrated how muskrats might interact with purple loosestrife in a manner detrimental to muskrats. Muskrats apparently find stems of purple loosestrife much less palatable then those of cattail, but they do cut purple loosestrife stems. As they forage they favor cattail stems, potentially shifting the competitive balance toward the less palatable purple loosestrife. The ability of muskrats to shift the competitive balance between cattails and purple loosestrife was corroborated by Rawinski [102] from observations of mixed stands where muskrats were present. At a particular site, muskrats removed entire patches of cattail, leaving purple loosestrife the only remaining emergent. Muskrats may further favor purple loosestrife seedling establishment following den construction. This activity can cause substantial soil disturbance that is rapidly colonized by purple loosestrife seedlings during lower summer water levels. Because of their ability to generate new vegetative growth, partially eaten purple loosestrife stems also represent potential new propagules, adding to its competitive advantage [23]. As community composition shifts from cattails to purple loosestrife dominance, habitat quality and subsequent muskrat carrying capacity apparently decline [129].
Conversion of wetland pasture to predominantly purple loosestrife is believed to reduce forage value for livestock and deer [128]. As purple loosestrife density increases and mature plants produce greater numbers of shoots, the woody nature of purple loosestrife stems diminishes forage value [118].
Purple loosestrife may have adverse effects on habitat of the threatened bog turtle, although details are scant [26,67].
Purple loosestrife invasion may be detrimental to production of natural and domestic wild rice in areas of the upper Midwest, particularly in commercial wild rice paddy operations where water level manipulation presents ideal germination conditions. Dense purple loosestrife infestations can also undermine the functionality of drainage waterways, such as irrigation ditches [118].
Water level manipulations in impoundments have been hindered by threat of purple loosestrife invasion. A 1000-fold increase in acreage containing purple loosestrife was noted over a 23-year period in a central New York wetland and the cause was speculated to be recurrent drawdown of impoundments [102]. In areas managed for waterfowl production, such as many federal and state wildlife refuges, water level drawdowns in impoundments may provide establishment opportunities for purple loosestrife. Drawdowns are often executed to encourage recruitment of plants valuable to waterfowl such as cattails, smartweed (Polygonum spp.) and wild millet (Echinochloa spp.) on exposed soils [90].
Invading purple loosestrife is being monitored in the middle Snake River corridor in Idaho for effects on stream channel dynamics. Purple loosestrife is colonizing gravel bars under low flow conditions. Once established, it appears able to withstand inundation and flowing water conditions better than native annuals. It is feared that persistent purple loosestrife plants may contribute substantially to sediment trapping, leading to gravel bar aggradation, closure of small channels, and despoiling of secure, predator-free island nesting habitat for local waterfowl [32].
Control: Land managers concerned about invasive purple loosestrife should focus on eliminating small, recently-established populations before tackling large, well-established populations. Buildup and persistence of purple loosestrife seed in the soil seed bank appears to be the most problematic, long-term obstacle in eradicating, or at least controlling purple loosestrife. Preventing seed production and seed bank accumulation within recently-established stands is a pragmatic goal, especially in the face of limited resources and knowledge [138,139]. Welling and Becker [138] demonstrated the potential difficulty managers face with attempts to exhaust seed banks in areas where purple loosestrife is well established, although not necessarily monodominant. Because seed dormancy is enforced by burial at relatively shallow (>0.8 inch (2 cm)) depth, and because purple loosestrife seed banks may contain thousands of seeds per square foot at these depths, even successful eradication of extant adult plants and new recruits from near-surface germinants may not suffice for successful long-term control. Even the ability to exhaust near-surface (<0.4 inch (1 cm)) seed banks by promoting germination and removing emergent seedlings is in question.
Any disturbance or management activity that fragments live stem or root tissue is likely to result in the spread, rather than containment of purple loosestrife [23,118]. Live stems that are dislodged and buried can give rise to new shoots via adventitious buds [23,129]. Carp may play an important role where they co-occur with purple loosestrife. Carp eat the roots of purple loosestrife, sometimes until the plants are dislodged and float away. These plants then become potential propagules if they lodge on suitable substrate [102].
Detection and control efforts may be hindered by purple loosestrife's propensity to occasionally remain dormant for an entire growing season. Some plants fail to generate aboveground shoots during a particular year, but exhibit normal growth from the same rootstock in preceding and following years [42,129].
Prevention: It is important to avoid management activities that may enhance the risk of purple loosestrife invasion and expansion. Examples of mitigative efforts are a) encourage establishment, growth, or perpetuation of native woody cover that might provide enough shade to depress or discourage purple loosestrife, b) minimize water level fluctuations in manipulated wetlands or waterways that might encourage establishment of purple loosestrife seedlings, especially early-season drawdowns that expose bare substrate, and c) avoid any form of stress or disturbance to extant native plant communities in susceptible areas, such as disturbing soil with heavy machinery, and where such activities are unavoidable, monitoring impacted areas to detect invaders [129].
Periodic, systematic monitoring of susceptible habitats is strongly encouraged [144]. Development of local populations, as expressed by percent biomass constituted by purple loosestrife, is roughly a logistic function through time. Initial rate of spread of local infestations is slowed when extant competition is strong. As a result, early detection and eradication of colonizing plants is highly preferred. Fortunately, early detection is aided by the tall, showy flower stalks and lengthy period of bloom. Once purple loosestrife becomes strongly established, with many (>10) flowering stems per rootstock, multiple clumps forming monospecific patches or stands, and establishment of a seed bank, eradication becomes more expensive, intrusive, and difficult [129].
Spread of purple loosestrife in natural areas likely has been accelerated by the development, sale and use of various loosestrife cultivars for horticultural purposes. Sale and utilization of ornamental loosestrife cultivars should be curtailed to prevent the risk of further dissemination into previously uncolonized areas. Cultivars are capable of contributing viable seed and pollen to wild populations, and claims of sterile hybrids have been shown to be mainly false [3,74,92].
As with most invasive species, public education plays an important role in preventing establishment and spread of purple loosestrife. Planting of loosestrife cultivars for horticultural purposes should be strongly discouraged. Individuals who frequent areas susceptible to invasion can aid in prevention by washing boots, clothing, equipment, etc. before exiting such areas, and should be encouraged to identify and report potential new infestations to authorities.
Integrated management: A single method may not be effective for long-term control or removal of purple loosestrife. Integrated management involves using several management techniques in a well-planned, coordinated and organized program. Many combinations of control methods can achieve desired objectives. Methods selected for a specific site will be determined by land-use objectives, desired plant community, extent and nature of infestation, environmental factors (nontarget vegetation, habitat types, climate, hydrology, etc.), economics, and effectiveness and limitations of available control techniques [103,114].
Cultural: Seeding of competitive vegetation in areas where bare soil has been exposed may be a useful mitigative measure. This may be especially helpful where presence of seed in the soil seed bank indicates potential for robust purple loosestrife regeneration. Experiments examining the effectiveness of seeding Japanese millet (Echinochloa esculenta) to reduce the impact of purple loosestrife recruitment have shown mixed results [80,140]. In addition to providing competition against purple loosestrife seedlings, Japanese millet may be used by waterfowl and is thought to represent a minimal threat of invasiveness, although it is not native to North America [129]. Seeding native species may provide a desirable postdisturbance community, but explicit tests of the competitive abilities of various native plants when seeded with purple loosestrife are lacking. Seeding of competitors should take place immediately following exposure of soil to maximize their competitive abilities [80].
Flooding infested areas by raising water levels for extended periods may eliminate purple loosestrife from impoundment sites [46]. Flooding duration is more likely to influence mortality than depth of flooding, but specific guidelines are lacking [9]. Persistent high water conditions can slow the growth and reproductive capacity of purple loosestrife and over several years may eliminate extant stands, but results are variable and interactions with other factors poorly understood [80]. In plots subjected to consistently high water levels (16 inch (40 cm) mean depth)), competition with narrow-leaved cattail significantly (P<0.001) reduced stem densities of purple loosestrife compared with flooded stands where purple loosestrife was the predominant species [101]. More research is needed to determine optimal flooding duration and factors that influence variability in the effect of flooding duration [9].
Effectiveness of flooding as a control measure may be enhanced by cutting purple loosestrife stems prior to raising water levels [80]. Cut material should always be removed from the site to prevent spread of vegetative propagules. The efficacy of flooding may be influenced by the presence of carp within contiguous waterways, although the ultimate effects are unclear. Carp may reduce purple loosestrife by grazing its roots or enhance its spread by disseminating vegetative propagules [102]. Carp are not native to North America and should not be introduced as a means to control purple loosestrife.
Consistent spring and early-summer flooding may inhibit purple loosestrife seedling establishment [9,137]. Flooding seedlings 0.8 to 4 inches (2-10 cm) tall for 9 weeks at depths up to 12 inches (30 cm) did not significantly (P<0.05) reduce mean stem densities. Most plants continued to grow, if slowly, while submerged, and plants which emerged above the surface quickly resumed rapid growth [52]. Established purple loosestrife plants can survive in deepwater emergent habitat, in part by development of aerenchymous (containing large intercellular air spaces) stem tissue that facilitates gas exchange in aquatic environments.
Several factors may hinder the effectiveness of controlling purple loosestrife by flooding. Managers may be constrained in their ability to manipulate water levels by the geologic profile of the site or by development along its margins. Substantial warm season evaporation can contribute to this problem. Sustained high water levels may be detrimental to desirable native emergent or shoreline vegetation. Once purple loosestrife has been killed, managers should consider species composition within the remnant seed bank, and the ensuing colonizing community, when water levels have been reduced. It is likely that purple loosestrife seedlings will recolonize the newly exposed soil and further management may be inevitable.
Physical/mechanical: Cutting stems or removing flower heads prior to seed dissemination can prevent local seed bank accumulation. Late-summer cutting appears to reduce vegetative growth more effectively than mid-summer treatments. However, cutting stems is unlikely to prevent perennial stem growth [46,102]. Cutting flower heads may be useful in preventing further seed production when primary control activities, such as herbicide application, require more than 1 season to completely eradicate purple loosestrife [13]. Cutting purple loosestrife stems underwater at various times in summer was ineffective [51].
Digging or hand-pulling plants is recommended for early infestations or a few scattered plants. Digging or pulling young plants in recently colonized areas can be effective in preventing establishment of dense, intractable stands and buildup of substantial seed banks. Early detection is important since established plants may rapidly become too large and deep-rooted for easy removal [102,129]. Because growing points of the plant are located on the root crown, removal of as much rootstock as possible is strongly encouraged [23,46]. Pulling entire plants is easiest when the soil is wet [102,131]. All pulled plant material should be removed from the site to prevent vegetative reproduction from discarded fragments [23]. Spot spraying individual plants with herbicide may be less time and labor intensive when infestations become too large for removal by pulling or digging [129].
Fire: See Fire Management Considerations.
Biological: The objective of biological control is to re-establish ecological relationships that have evolved between purple loosestrife and its native predators in order to suppress invasive populations and reduce harmful impacts. Potential advantages of biological control are cost effectiveness at large scales, sustainability, and benign effects in the nontarget environment [22,131]. The Nature Conservancy's Weed Control Methods Handbook provides a comprehensive discussion of considerations and safety issues in developing and implementing a biological control program.
Plant communities where purple loosestrife is found are similar in North America and Europe. Because native insect herbivory inhibits purple loosestrife performance in Europe, it is hoped introductions of European insect herbivores may work to reduce the competitiveness of purple loosestrife in North America, while releasing native plants from suppression [18,19].
The following table lists non-native insects released in North America to control purple loosestrife:
Control Agent Mode of Action Release Sites Galerucella calmariensis (beetle) Larvae and adults feed on foliage and flowers MB, ON Galerucella pusilla (beetle) Larvae and adults feed on foliage and flowers [18] MB, ON, WA [29,31,97] Hylobius transversovittatus (weevil) Larvae and adults feed on roots [17] WA [97] Nanophyes marmoratus (weevil) Larvae feed on flowers and adults feed on foliage and flowers [21] MB [49]Galerucella beetles have been the most effective biocontrol agents used against purple loosestrife in North America thus far [29,62,97]. G. calmariensis and G. pusilla are similar in appearance and habit and are most effective when released together, and both species appear to be unaffected by exposure to the herbicides glyphosate and triclopyr [75,76]. Because of "dramatic" success at some Galerucella release sites, release of other agents should focus on sites where Galerucella have been ineffective. In Europe, H. transversovittatus herbivory on purple loosestrife is strongest in the northern range of the plant, indicating that higher latitude sites may be a good choice for its release in North America [50].
Myzus lythri, a European aphid that has probably been present in the Eastern United States since the early 1930's, might become an effective biological control agent. It has a host-alternating life cycle, utilizing loosestrife and Epilobium spp. in summer and Prunus spp. as primary hosts the rest of the year. Populations of M. lythri could be manipulated to impact local purple loosestrife populations by mass-rearing bugs for targeted early-spring release and/or by planting Prunus spp. near targeted sites [134].
Research examining the potential use of pathogenic fungi as biocontrol agents is ongoing [91].
Chemical: A variety of herbicides are effective at controlling purple loosestrife in infested areas. Below is a list of herbicides that have been used effectively against purple loosestrife in North America, as well as a brief discussion of important considerations regarding their use. This is not intended as an exhaustive review of chemical control methods. For more detailed information regarding appropriate use of herbicides in natural areas against this and other invasive plant species, see The Nature Conservancy's Weed Control Methods Handbook.
Chemical Considerations 2,4-D [13,90,118,140] Mixed results against purple loosestrife; harmful to dicots, but little impact on neighboring monocots Triclopyr [12,38,61,89,118] Generally effective at killing purple loosestrife; results are variable with spray volume; selective against dicots Glyphosate [12,80,102,104,118,122,131] Highly effective against purple loosestrife; specific formulations available for use in aquatic environments; also damages or kills most other plants which it contacts Imazapyr [11] Effective against purple loosestrife; negatively impacts cattailA serious challenge to controlling purple loosestrife infestations with herbicides is preventing its re-establishment from the seed bank. In the presence of large purple loosestrife seed banks, removal of a considerable fraction of extant vegetation (weed or otherwise) can result in a dense monoculture of purple loosestrife seedlings. The result may be a worse infestation than was originally present [90]. Broadcast application of broad-spectrum herbicides, such as glyphosate, will likely result in widespread exposure of bare substrate and a dense, monotypic stand of purple loosestrife seedlings [118]. By carefully targeting glyphosate spray application to only purple loosestrife, damage to nontarget plants can be minimized. Continued careful treatments over several years can eventually reduce dense populations of purple loosestrife to minimal levels while promoting native plants [104,122]. Native plants are not just inherently valued, but can also provide competition against inevitable purple loosestrife recruitment from existing seed banks [118].
An apparent tradeoff exists when determining the best time to treat adult stands with herbicides. Managers must attempt to balance preventing seed production in established plants with treatments early in the growing season and preventing establishment of a viable new stand of purple loosestrife seedlings by delaying treatments long enough to inhibit recruitment. By conducting herbicide treatments on adult plants late in the growing season, newly established seedlings may not develop sufficiently to survive winter [89]. Late-summer herbicide application also appears to reduce negative effects on desirable native plants [80]. Rawinski [102] found that glyphosate application during late-bloom (mid-August in central New York) period, compared with late-vegetative (mid-June) period, resulted in fewer loosestrife seedlings the following season and increased presence of naturally established, beneficial plants such as shallow sedge (Carex lurida), rice cutgrass (Leersia oryzoides), smartweed and marsh seedbox (Ludwigia palustris). Late-season application of glyphosate in Minnesota wetlands tended to reduce cattail mortality compared with mid-summer treatments, perhaps because the onset of cattail senescence reduced herbicide uptake [12].
Another tradeoff exists between spray volume and target vs. nontarget effects. Purple loosestrife in Wisconsin was examined for response to variation in spray coverage of glyphosate (Rodeo at 1.5%). Individual genets were spot treated in mid-September and received either low (10-25% leaf area coverage), medium (40-60%), or high (75-90%) dosages. Reduction in adult purple loosestrife density was greatest in the high dosage treatment (90-100% reduction) and lowest in the low dosage treatment (75-90% reduction). Surviving purple loosestrife plants in all treatments were greatly reduced in size and vigor. Because glyphosate is nonselective in its effect, survival of nontarget vegetation was also closely related to dosage. High dosage treatment resulted in dense stands of purple loosestrife seedlings with little to no interspecific competition. In contrast, low dosage treatment resulted in high survival rates of desirable perennials and greatly reduced germination of purple loosestrife seedlings. Effective long-term control of purple loosestrife with glyphosate might best be achieved using low-dosage spot applications and conducting followup treatments in subsequent years as necessary [104].
To minimize non-target effects, managers in Michigan have developed a cut-and-herbicide method for purple loosestrife control. They propose cutting plants high on the stem (just below infloresence), allowing them to continue growing and better absorb the applied herbicide throughout the entire plant. Cutting too low apparently risks forcing the plant to "give up" on the leader and instead producing new ramets from the rootstock. Sponge applicators have been developed that limit contact between chemicals and nontarget plants [131]. These methods may be particularly useful in areas where mitigation of damage to indigenous species is important. Encouraging competition from extant native plants often helps reduce the vigor of invasives. For more detailed information regarding these methods, see Tu [131] and the TNC Weed Control Methods Handbook.
While purple loosestrife invasion is often reported as detrimental to wetland-bird habitat, some evidence indicates little to no harmful effect. American coot, pied-billed grebe, black-crowned night heron, American goldfinch and gray catbird have all been observed nesting in purple loosestrife stands [2,102]. Red-winged blackbirds preferentially nest in purple loosestrife over cattails [101,142]. American goldfinch construct nests in purple loosestrife, utilizing the relatively stable stalks to attach nests above the ground or water surface [68]. Pied-billed grebes use dead purple loosestrife stems as nest substrate in habitat with standing and emergent vegetation [77]. In a 2-year survey of birds in wetlands surrounding Lake Huron's Saginaw Bay in eastern Lower Michigan, swamp sparrow nests were most abundant in areas of purple loosestrife dominance [142].
Although purple loosestrife, with its tiny seeds, has been assumed to provide little to no food for birds [129], there are several reports of ducks and red-winged blackbirds consuming purple loosestrife seeds [2], and a report of damage to experimental seedling plots in England caused by ring-necked pheasants and pigeons [113].
In a host-specificity test of potential biological
control agents for purple loosestrife, Blossey and Schroeder [21] included 13
plant species said to "occur in the same habitat" as purple
loosestrife and were "of wildlife importance." Although these species are not necessarily distributed
homogeneously or systematically across the North American
landscape, they likely represent a reasonable sample of typical plant
associates. These species were common cattail (Typha
latifolia), broadfruit bur-reed (Sparganium eurycarpum), broadleaf
arrowhead (Sagittaria latifolia), annual wildrice (Zizania aquatica),
Olney threesquare (Scirpus americanus), hardstem bulrush (Scirpus acutus), longhair sedge (Carex comosa), sandbar willow (Salix
exigua), curly dock (Rumex crispus), longroot smartweed (Polygonum
amphibium), lambsquarters (Chenopodium album), cursed buttercup (Ranunculus
sceleratus) and St. Anthony's turnip (Ranunculus bulbosus).
Classifications describing plant communities in which
purple loosestrife is a dominant species are:
New York [105]
Washington [58]
Breeding system: Purple loosestrife is a tristylous species (3 different style lengths), usually in a 1:1:1 ratio, indicating sexual reproduction is probably its most important means of regeneration [9]. It is primarily an outcrosser, as self-pollination in purple loosestrife is rare, and has been shown to reduce seed production [111].
Pollination: Purple loosestrife is insect pollinated. Most reports indicate honeybees are the main pollinators [43,73]. Others include bumblebees [72,73], leaf-cutter bees and carpenter bees [72], as well as a variety of butterflies [72,73]. Hummingbirds have been observed taking nectar from purple loosestrife in British Columbia [98], although pollination by hummingbirds is undocumented.
Seed production: Purple loosestrife produces an immense number of seeds. Estimates of seed production rates range from just over 100,000 seeds per plant for young plants with single stems [111], to over 2.5 million seeds per plant for established plants with an average of 30 stems per plant [129]. Although perennial, purple loosestrife is capable of producing viable seed during its 1st growing season [116]. Seed output is largely a function of plant age, size, and vigor [129]. Shoots growing in relatively dense stands tend to produce fewer and smaller inflorescences than those growing in more open areas [102].
Seed dispersal: Because seeds are small and light they are thought to be dispersed, at least in part, by wind [53,111]. However, Thompson and others [129] report observations that seedling densities decline sharply within a 33 foot (10 m) perimeter of the parent plant and seedlings are often distributed downslope from the parent plant rather than downwind, suggesting a limited role for wind dispersal. Dispersal via moving water is also likely [53,118,119]. Seeds and cotyledon stage seedlings are reportedly buoyant [9], although there are reports that purple loosestrife seeds don't float [119]. Floating seeds may disperse to suitable sites for establishment. Seeds that sink may germinate while submerged, then rise to the surface and drift to suitable sites for establishment [129]. Seeds may be transported in fur of mammals, plumage of waterfowl, mud attached to footgear, vehicle treads or cooling systems of outboard motors [53,128,129]. Thompson and others [129] also suggest birds may deposit ingested seeds in areas where wind or gravity-mediated dispersal seems unlikely.
Seed banking: Given its high seed output and ability to produce seed in its 1st growing season, purple loosestrife can establish substantial soil seed banks. Seeds may remain viable for at least 2 to 3 years [102,111], although the long-term viability of seeds stored in the soil seed bank remains under investigation [139]. Seeds may remain viable even when subjected to saturating conditions. Viability of seeds that were stored underwater was tested at 4-month intervals under ideal germination conditions. Germination declined from an initial rate of 99% to 93% after 1 year and 80% after 2 years [102].
Purple loosestrife has the potential to dominate the soil seed bank where it becomes well established. Soil samples taken from within purple loosestrife stands in emergent wetlands in southeastern Minnesota contained an average of 37,963 purple loosestrife seeds per ft2 (410,000 /m2) in the top 2 in (5 cm) of soil. Seeds were distributed within this entire profile and seed density increased with proximity to the soil surface. Under greenhouse conditions chosen to promote germination, and using soil samples from the above source spread 0.4 in (1 cm) deep, recruitment failed to exhaust the seed bank [138,140]. From the same experiment, purple loosestrife seedlings were found in 91% of untreated (no herbicide) 6.6 x 6.6 feet (2 x 2 m) quadrats, the most frequently encountered species in the soil seed bank [140].
Germination: Germination is greatest in unshaded, wet soils, with temperatures >68 degrees Fahrenheit (20º C) [20]. Shamsi and Whitehead [111] demonstrated germination is constrained at low temperatures between about 50 to 59 degrees Fahrenheit (10°-15° C), and no germination occurred below 57 degrees Fahrenheit (14° C). Experimental evidence indicates seed dormancy may be enforced by burial, with germination response decreasing linearly (p = 0.001, r2 = 0.89) from 90% at the soil surface to 0% at 0.8 in (2 cm), even under conditions known to promote germination in wetland plants [138]. Any disturbance that redistributes seeds to within the upper 0.8 inch (2 cm) of soil is likely to promote germination. Although light exposure is a prerequisite for germination, length of exposure does not appear important [112]. Purple loosestrife seeds are capable of germinating underwater [64].
Seedling establishment/growth: Favorable recruitment conditions are largely a function of disturbance that creates areas where little to no vegetation is present [99]. Estimates of maximum initial seedling density vary greatly, from 926 to 1,852 foot-2 (10,000-20,000 m-2) on bare open mudflats [102] to 2.8 to 4.6 foot-2 (30-50 m-2) in vegetated semiflooded wetlands. In areas where large numbers of seeds are present in the seed bank, small changes in area favorable for establishment can yield large fluctuations in recruitment [1].
In order to begin successful establishment, floating seeds or propagules must settle on moist soil [129]. Purple loosestrife can establish in soil beneath standing water [64].
Growth is limited by cold temperature and is considerably slowed at around 46 to 50 degrees Fahrenheit (8-10º C) [141]. Light availability can also limit growth and development. Under diminishing light intensities, vegetative growth is slowed, the numbers of flowers, fruits, and seeds per fruit are fewer, and the average dry weight of fruits declines, but there is no change in average dry weight of individual seeds [112]. Growth is also affected by day length. Shamsi and Whitehead [112] found leaf area and plant dry weight were significantly (P<0.05) reduced when plants were subjected to a 9-hour photoperiod compared with a 16-hour photoperiod. Plants in the 9-hour treatment grew in a comparatively flattened, semi-prostrate condition.
Asexual regeneration: The rootstock is the main organ of perennation, and unaided wide vegetative spread is unlikely. New shoots arise from buds at the top of the rootstock [111]. Root crowns expand annually to accommodate increasing numbers of shoots, but may reach maximum growth at around 20 inches (0.5 m) in diameter [129].
Purple loosestrife can consistently resprout in response to aboveground damage, often fairly rapidly. A greenhouse experiment showed 91% of clipped seedlings resprouted within 42 days [39]. Live stems that are dislodged and buried can give rise to new shoots via adventitious buds [23,129].
Purple loosestrife displays many characteristics typical of pioneer species, such as rapid maturity, high seed production, tolerance of nutrient-poor environments, and high germination success. Yet North American populations, once established, also are potentially long-lived (22+ years), capable of growing to a relatively large size, and have shown the propensity for near-continuous, low-level recruitment in the absence of large-scale disturbance [1,129]. While evidence is somewhat limited, it is speculated natural mortality rates in adult plants are quite low [1].
Purple loosestrife, once established, can persist within a site for relatively long periods, even in the absence of frequent disturbance. After examining purple loosestrife population structure within several different communities in eastern Massachusetts, Anderson [1] concluded low levels of nearly-continuous recruitment are likely to occur in areas where mature plants (and the inevitable prodigious purple loosestrife seed bank) are present. Additionally, this trend is punctuated by occasional disturbances that provide conditions suitable for short-lived recruitment episodes in which relatively large cohorts of new plants are established.
But there is some question regarding the view that purple loosestrife inevitably dominates invaded sites in virtual monotypic stands. Anderson [2] points out that in a widely cited review by Thompson and others [129], estimates of the proportion of stand biomass attributed to purple loosestrife, which ostensibly increased over time following establishment, may instead have been attributable to increases in the number of stems per genet rather than greater numbers of individual plants. The number of annually produced stems per single genetically distinct plant has been shown to be a good predictor of the age of that individual [1]. Anderson [2] also notes observations described in Thompson and others [129] were strictly visual assessments, and since no hard data was collected, there is no way to definitively ascertain what, if any, changes in biomass distribution among species may have occurred.
In its native range, European populations of purple loosestrife may also form large monospecific stands following pregrowing season disturbance, but are prone to invasion by other species soon after stand establishment [110,111]. Whitehead [141] described the gradual yielding of monospecific stands of purple loosestrife to mixed species communities in England as being due to slow growth of purple loosestrife during periods of cool spring temperatures compared with competitors possessing low-temperature growth capabilities such as cattails or reeds (Phragmites spp.) It is likely that an aggregate of factors act to limit purple loosestrife site dominance in its native habitats [118].
Thompson and others [129] have reviewed several historical accounts of purple loosestrife stands, both in its native Europe and elsewhere. They determined that while purple loosestrife seldom maintains strong community dominance in native (European) habitats, it commonly forms dense, long-lasting, virtually monospecific stands in areas where it is not native, especially temperate North America. They considered 3 factors that could possibly account for this phenomenon: 1) the absence of many key insect predators that effectively reduce competitiveness of European purple loosestrife plants, 2) predominance of the muskrat in its native North American habitat and the impact of its selective Foraging behavior on cattails (see Importance to Livestock and Wildlife or Impacts and Control), and 3) the possibility that North American purple loosestrife may have evolved adaptive traits which make it more vigorous and competitive than its European relatives.
Many factors are likely to affect the ability of purple loosestrife to form and maintain extensive monodominant stands in North American wetlands. Characteristics particular to certain classes of habitat may lead to monodominance. Auclair and others [8] have noted some trends in 2 distinct plant communities of Huntington Marsh, located along the St. Lawrence River near the junction of the Quebec, Ontario and New York borders. In the emergent aquatic community, the dominant emergent taxa tended to exclude each other, resulting in a mosaic of nearly monospecific communities. In particular, river bulrush (Schoenoplectus fluviatilis), common reed (Phragmites australis) and narrow-leaved cattail (Typha angustifolia) displayed this phenomenon. In contrast, sedge meadow communities were much more diverse and lacked the dominance and segregation of species. Instead they demonstrated subtle gradients in composition that were generally difficult to discern.
The nature of particular disturbance events may also impact initial floristics and subsequent successional trajectories. For instance, the relative competitiveness of purple loosestrife seedlings following disturbance may depend upon when initiation of the new seedling community occurs within the growing season. Because purple loosestrife growth rates are closely linked to day length [112], early summer establishment of a seedling cohort or community, compared with late summer establishment, is more likely to result in a monospecific stand of purple loosestrife because purple loosestrife seedlings will be more competitive [102].
More research is needed to help elucidate the means and extent to which purple loosestrife alters successional trajectories and community dynamics. Long-term studies that examine preinvasion vs. postinvasion data would be particularly helpful.
The currently accepted scientific name of purple loosestrife is Lythrum salicaria L. (Lythraceae) [57,60,71].
Purple loosestrife will hybridize with
European wand loosestrife (Lythrum virgatum) and winged loosestrife
(Lythrum alatum) [3,92]. A
number of different horticultural cultivars have been developed from purple
loosestrife and wand
loosestrife. Although some are purported to be sterile, crosses within and
between cultivars and wild Lythrum spp. are often compatible, and
identification of cultivars and crosses is problematic [92,118].
Lythrum salicaria or purple loosestrife[2] is a flowering plant belonging to the family Lythraceae. It should not be confused with other plants sharing the name loosestrife that are members of the family Primulaceae. Other names include spiked loosestrife and purple Lythrum. This herbaceous perennial is native to Europe and Asia,[3] and possibly Australia.[4]
The generic name Lythrum is derived from the Greek ‘lythron’, meaning blood, in reference to the flower colour in some species.[5] However, Pliny (A.D. 23-79) stated that Lythrum is named for Lysimachus, an army general and friend of Alexander the Great. ‘’Lysimachus’’ is derived from the Greek ‘’lysis’’, meaning ‘’loosing’’ and ‘’mache’’, meaning strife.[6]
The specific epithet salicaria derives from the similarity of the leaves to those of willows or Salix species.[6]
The English word loosestrife first appeared in written form in the 16th century. It was coined by English naturalist William Turner in his 1548 work A New Herball, in which he states: "[The plant] may in englishe be called red loosestryfe or purple loosestryfe."[6]
Purple loosestrife was referred to several times as ‘Long Purples’ by John Clare, such as in his 1821 poem The Wildflower Nosegay:[7]
"Where on the water op’d the lily buds
And fine long purples shadow’d in the lake..."
— Volume II of The Village Minstrel and Other Poems, John Clare, pg. 133
Lythrum salicaria can grow 1–2 m (3 ft 3 in – 6 ft 7 in) tall, forming clonal colonies 1.5 m (4 ft 11 in) or more in width, with numerous erect stems growing from a single woody root mass. The stems are reddish-purple and square in cross-section. The leaves are lanceolate, 3–10 centimetres (1–4 in) long and 5–15 millimetres (3⁄16–9⁄16 in) broad, downy and sessile, and arranged opposite or in whorls of three.
The flowers are reddish purple, 10–20 millimetres (1⁄2–3⁄4 in) in diameter, with six petals (occasionally five) and 12 stamens, and are clustered tightly in the axils of bracts or leaves; there are three different flower types, with the stamens and style of different lengths, short, medium or long; each flower type can only be pollinated by one of the other types, not the same type, thus ensuring cross-pollination between different plants.[2][8][9] The flowers are visited by many types of insects, and can be characterized by a generalized pollination syndrome.[10]
The fruit is a small 3–4 millimetres (1⁄8–5⁄32 in) capsule[11] containing numerous minute seeds. Flowering lasts throughout the summer. When the seeds are mature, the leaves often turn bright red through dehydration in early autumn; the red colour may last for almost two weeks. The dead stalks from previous growing seasons are brown.[2][8][9]
L. salicaria is very variable in leaf shape and degree of hairiness, and a number of subspecies and varieties have been described, but it is now generally regarded as monotypic with none of these variants being considered of botanical significance. The species Lythrum intermedium Ledeb. ex Colla is also now considered synonymous.[2][9][12]
Lythrum salicaria is native to Europe, temperate Asia and northwest Africa. It is also naturalised in many temperate parts of the world, including parts of southern Africa, North America, and South America.[8][12][13][14][15] The species was thought to be a recent introduction to Australia, but pre-colonial pollen samples have been found in New South Wales, indicating another route of introduction;[16] the question is considered unresolved by Australian authorities.[4]
Found in ditches, wet meadows and marshes and along sides of lakes.[17] In North America, purple loosestrife can invade sedge meadows.[18]
The flowers are pollinated by long-tongued insects, including bees and butterflies.[9]
A number of insects use Lythrum salicaria as a food resource.
The black-margined loosestrife beetle Galerucella calmariensis is a brown beetle with a black line on its thorax. The adult feeds on the leaves of the plant, producing characteristic round holes. Its larvae destroy tender leaf buds and strip the tissue from the leaves. The golden loosestrife beetle Galerucella pusilla is nearly identical to G. calmariensis, but usually lacks the black thoracic line. Its feeding habits are also quite similar to the other leaf beetle.
The loosestrife root weevil Hylobius transversovittatus is a large red nocturnal weevil, which spends its nights feeding on leaves and leaf buds. The larvae emerge from their eggs and immediately burrow into the root of the plant, which they feed on continuously for over a year. This root damage stunts the plant's growth and ability to create seeds. If several larvae inhabit the same root, the plant can be killed.
The loosestrife flower weevil Nanophyes marmoratus is a tiny weevil which lays a single egg in each flower. When the larvae emerge they eat the flowers' ovaries, and the plant is unable to create seeds. The larvae usually proceed to hollow out the flower buds and use them as safe places to pupate.
Caterpillars of the engrailed moth (Ectropis crepuscularia), a polyphagous geometer moth, also feed on purple loosestrife.
It has been used as an astringent medicinal herb to treat diarrhea and dysentery; it is considered safe to use for all ages, including babies.[19] It is also cultivated as an ornamental plant in gardens, and is particularly associated with damp, poorly drained locations such as marshes, bogs and watersides. However, it will tolerate drier conditions. The flowers are showy and bright, and a number of cultivars have been selected for variation in flower colour, including:
The cultivars ‘Blush’[21] with blush-pink flowers, and 'Feuerkerze'[22] with rose-red flowers have gained the Royal Horticultural Society's Award of Garden Merit.[23]
It has also been introduced in many areas of North America by bee keepers, due to its abundance of flowers which provide an important source of nectar.[24]
The purple loosestrife has been introduced into temperate New Zealand and North America where it is now widely naturalised (spreading on its own) and officially listed in some controlling agents. Infestations result in dramatic disruption in water flow in rivers and canals, and a sharp decline in biological diversity as native food and cover plant species, notably cattails, are completely crowded out, and the life cycles of organisms from waterfowl to amphibians to algae are affected.
A single plant may produce up to 2.7 million tiny seeds annually.[25] Easily carried by wind and water, the seeds germinate in moist soils after overwintering. The plant can also sprout anew from pieces of root left in the soil or water. Once established, loosestrife stands are difficult and costly to remove by mechanical and chemical means.
Plants marketed under the name "European wand loosestrife" (L. virgatum) are the same species despite the different name. In some cases the plants sold are sterile, which is preferable.
In North America, purple loosestrife may be distinguished from similar native plants (e.g., fireweed Chamerion angustifolium, blue vervain Verbena hastata, Liatris Liatris spp., and spiraea (Spiraea douglasii) by its angular stalks which are square in outline, as well as by its leaves, which are in pairs that alternate at right angle and are not serrated.
Purple loosestrife provides a model of successful biological pest control. Research began in 1985 and today the plant is managed well with a number of insects that feed on it. Five species of beetle use purple loosestrife as their natural food source, and they can do significant damage to the plant. The beetles used as biological control agents include two species of leaf beetle: Galerucella calmariensis and Galerucella pusilla; and three species of weevil: Hylobius transversovittatus, Nanophyes breves, and Nanophyes marmoratus. Infestations of either of the Galerucella species is extremely effective in wiping out a stand of purple loosestrife, defoliating up to 100% of the plants in an area. The moth Ectropis crepuscularia is polyphagous and a pest species itself, and unsuitable for biological control.[26]
Lythrum salicaria or purple loosestrife is a flowering plant belonging to the family Lythraceae. It should not be confused with other plants sharing the name loosestrife that are members of the family Primulaceae. Other names include spiked loosestrife and purple Lythrum. This herbaceous perennial is native to Europe and Asia, and possibly Australia.
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