Ageratum houstonianum Mill.

China: Ageratum houstonianum flowers and fruits all year round.
Madagascar: Ageratum houstonianum flowers throughout the year, mainly from March to June.
Zimbabwe : Ageratum houstonianum flowers and fruits from October to May.

Ageratum houstonianum prefers damp and shady conditions. It grows along roadsides, in disturbed areas and along river banks.
 
China: Species present in meadows, roadsides, slopes in valleys, cultivated and naturalised, at altitudes of 100-1500 m.
Fiji: Occasionally cultivated and also naturalised at altitudes from sea level to 1,127m in open areas, along roadsides, river banks, and on slopes and ridges of cleared uplands.
Hawaii: Naturalised in a wide variety of disturbed habitats, particularly along tracks and roads, from 40 to 1,300 m.
Madagascar: The species thrives on usually quite fertile soils such as ferrallitic, more or less humid of the slopes and terraces, under a humid (eastern side of the island) and sub humid climate of upland, moist soil but well drained. Sunny or lightly shaded grounds, in cropping systems based on vegetables, cassava, slash and burned rainfed rice in extensive or semi-intensive system. Agro-ecological zones in Madagascar are the central highlands and the eastern side of the island.
Mauritius: Common in the sugar cane fields and on roadsides, especially in the per humid area.
New Guinea: Ageratum houstonianum is established in a few localities above 1000 m.
Reunion: Absent.
South Africa: Ageratum houstonianum is a crop weed.
West Indies: Ageratum houstonianum is mentioned as an ornamental plant but its presence in natural or agricultural environments remains to be confirmed.
Zimbabwe: Ageratum houstonianum grows along roadsides, in disturbed environments and beside watercourses, at altitudes of 1,000 to 1,700 metres.

Global harmfulness
 
Ageratum houstonianum is a common weed of annual crops; can invade perennial crops such as sugar cane. Occasionally, there can be a significant problem for cotton where the achenes transported by wind contaminating the fiber. The seeds are also referred to stick in the throats of cows and can choke and kill them.
 
Local harmfulness
 
South Africa: Crop weed. Present in the provinces of Eastern Cape, Kwa-Zulu Natal, Limpopo and Mpumalanga. Considered as species of Category 1. Nemba - Category 1B
Madagascar: Fairly common with variable abundance in cassava crops, perennial crops, rice slashing and burning, vegetable crops, on roadsides and cultures.
Mauritius: Present but not abundant. It competes severely with young canes when present in large numbers.
Reunion: Absent.
 

Ornamental: Plant frequently used as an ornamental plant.
Medicinal: Anti-inflammatory, healing

Global management
 
Manual weeding of Ageratum houstonianum is not recommended because of its allergenic effects. Herbicides can be used in crops in pre-emergence and / or post-emergence directed.
 
Local management
 
South Africa: In Kruger National Park, Ageratum houstonianum is controlled by manual weeding
Madagascar: Use the angady weeding or manual weeding to fight against this weed in vegetable crops, cassava or potato, perennial crops and rice cultivation on slash burning. Soil moisture can limit the effectiveness of manual weeding; manual weeding can cause an allergy phenomenon.
Mauritius: Weeds can be controlled manually by uprooting or by herbicide application.
Reunion: Absent.

Herbarium pictures ReCOLNAT: https://explore.recolnat.org/search/botanique/simplequery=Ageratum%2520houstonianum