Platycrater arguta

In 2019, before the pandemic, I started a new Hydrangea tome since seismic taxonomic, breeding and cultivar introduction advances occurred after the publication of my Hydrangeas for American Gardens in 2004. Aside from the avalanche of new cultivars, especially prevalent among H. arborescens, H. macrophylla and H. paniculata, one of the most significant changes is the merging of several Hydrangeaceae genera into Hydrangea. Decumaria, Dichroa, Pileostegia, Platycrater and Schizophragma are now Hydrangea species according to Taxon 64(4): 741-753 (2015). Though these changes will not be universally adopted, I noticed the Germplasm Resources Information Network (GRIN), maintained by the USDA-ARS, has embraced them. Through six editions of my Manual, Platycrater was not included. The Hillier Manual of Trees and Shrubs (2019, 9th edition) has never listed the species through nine editions. It is not a household garden word and checking internet sites is limitedly available. The past few years, I collected several cultivars and I’m on the hunt for others. I showed a plant in full flower to a nursery friend in 2020 and he commented it would make a great hanging basket plant. I believe the species and cultivars have much more to offer than this, so let’s dive in.

In the Dirr garden, the species overwintered without stem injury in containers after the 2019-20 winter, when the lowest temperature in Athens was 21.1°F. These plants were in leaf in late March with flower buds evident in April and in full expansive flower in June. The emerging leaves are bright green, darkening with maturity, turning yellow in November-December in Zone 8. Leaves are opposite, 3-6 inches long, 1½ to 3½ inches wide (often less in width), petiole ½ to 2½ inches long (petiole on my original plant is ¼-inch long), acuminate apex, narrow cuneate base, the blade lanceolate to elliptic, with 7-9 veins on either side of the midvein, the margins roughly serrate to serrulate.

Flowers develop from previous year’s wood and on the new growth in June-July, continuing sporadically for a time, never overwhelming, yet elegant and refined. The original plant in my possession developed flowers from previous year’s stems as well as new growth. Inflorescence is a terminal panicle (may be corymb) with multiple semi-pendulous flowers. Each flower composed of 3 to 4 white sepals, 1-1¼ inches in diameter, broad ovate, and fused from the base to the middle, forming a triangle or square outline. Flowers may be fragrant or have no odor. The yellow stamens are positioned in the center of the reflexed sepals. Platy means flat and crater means bowl in reference to the flowers while arguta means silver, referring to the white flowers. I noticed all manner of insects visiting the flowers. The fruit is a striate (parallel lines), two-valved capsule, long styles resembling antennae, each capsule with tiny dark brown seeds.

The species is best grown in filtered light with uniform moisture and good drainage. Atlanta Botanical Garden-Gainesville grows the species in shade and full sun where it is in leaf 2-3 weeks ahead of H. macrophylla. I observed the Atlanta Botanical Garden-Gainesville plant in full flower in early June; the foliage as fresh as a daisy (no flowers) in October. The species has exhibited significant heat tolerance which is unexpected considering its native habitat in the wild. Foliage is exceedingly clean with no evidence of mildew or Cercospora at Premier Introductions, Inc. Cuttings root in 4-6 weeks when treated with 0.15% KIBA quick dip and placed under mist. On June 24, 2020, single-node cuttings were treated with 0.15% KIBA quick dip, placed in medium under mist and had rooted by July 28, 2020. Seeds should be processed similarly to H. arborescens and H. macrophylla, i.e, well-drained, peat-based germination mix with seeds surface sown and watered thoroughly. I was told seedlings require two years from seed sowing before flowering.

Cultivars

‘Pink Moon’ from Issima Works (Compton, Rhode Island) has larger flowers, larger number of sterile flowers, and deeper pink sepal color than ‘Rosea’. The color is on the underside of the sepals so somewhat minimized for the viewer. Flowers are fragrant.

‘Rosea’ has pink flowers and is offered by Broken Arrow Nursery, (Hamden, Connecticut). Was sent a few cuttings by Adam Wheeler, horticulturist, and the buds are bright pink.

Variegated form is not exactly dramatic as leaves are blotched/streaked/stippled gray and green.

I procured the species for possible hybridization work with other Hydrangeaceae members. Was not sure this is possible until reading Rinehart et al., Proc. SNA Res. Conf. 50: 656-659 (2005), where the authors showed affinities among Platycrater arguta, H. involucrata and H. aspera. Kind of an intriguing idea. Was told successful crosses were made with H. aspera but have yet to confirm. The large-flowered white and pink cultivars lead me to believe it could be a stand-alone ornamental flowering shrub. There are breeding opportunities using the cultivars listed above. It could prove a pretty garden plant as a textural contrast with large foliaged hydrangeas and other shade loving plants and would function as an elegant container, and perhaps hanging basket plant.