Ageratina riparia aka Mistflower

Ageratina riparia is a perennial, a plant that lives for years. It belongs to the daisy family, Asteraceae. Its home range is in Mexico, from Michoacán to Veracruz and Oaxaca. Kew places it in the seasonally dry tropical biome. This means a tropical zone with a dry season. Its older name is Eupatorium riparium. People grow it for its looks, but it often escapes gardens. It now grows in many places beyond its home range.

The plant grows upright or sprawls across the ground. Reports give its height as about 1.6–3.3 feet [0.5–1 metre]. Its creeping stems root where their nodes touch the ground. These stems weave together to form a thick mat. The mat can leave little room for other plants. Insects carry pollen between its flowers. Each plant can make as many as 100,000 seeds a year. Wind blows the light seeds away from the parent plant.

It can grow in shade, but frost harms it. In Southeast Asia, it grows in damp, shaded sites. These include stream banks, field edges and slopes. It also grows among tea crops and in forest clearings. To grow new plants, sow seed on the surface. Keep the seed moist. Light helps seeds sprout; buried seeds are less likely to sprout. A very thin soil cover does less harm than deep sowing. Up to three-quarters of fresh seed may sprout within one week.

Its use in gardens comes with a serious weed risk. Thick stands crowd out native plants and slow their return. It also spreads into pastures in some wet upland areas. This can leave the land able to feed fewer grazing animals. Fallen leaves release chemicals that can curb nearby crop growth. Parts of North America list it as a noxious weed. It causes weed problems in several Asian countries, Australia and Hawaii. Weed killers alone can struggle to kill it. In Hawaii, white smut fungus has helped control it.

People also grow it as green manure and to hold soil. Some gather wild plants for local use as medicine. Reports say it can act against fungi. No one knows of any edible uses. Reports from southern Queensland link horse deaths to eating this plant. The suspected harm includes lung damage. It also includes lasting swelling in the small airways. These reports call for care, though the source says the link remains suspected.