Mistflower (Ageratina riparia
) is a low-growing, multi-branched, spreading herbaceous perennial that is native to Mexico and the Caribbean. It has been introduced to subtropical and tropical regions worldwide. A. riparia
is often found growing in sheltered, moist areas, such as damp hillsides among rocks, waterways, roadsides, disturbed sites, pastures, gullies, rainforest clearings, and shaded deep creek banks. A. riparia
is considered to be an aggressive, invasive weed in many regions of the World, where it threatens many rare and endangered native plants.
has numerous spreading stems and oppositely arranged leaves that are relatively narrow with toothed margins. Dense, flat clusters of irregular fuzzy small, white flowers develop between July and October. A. riparia
looks very similar to crofton weed (Ageratina adenophora)
. The difference is that crofton weed is an upright (erect) plant with relatively broad, diamond-shaped (rhomboid) or slightly triangular leaves. It's young stems are densely covered in sticky (grandular) hairs.
All parts of A. riparia
are toxic to horses, resulting in Numinbah horse sickness
. The specific toxin responsible for poisoning has not yet been identified, however it has a cumulative effect on horses. Symptoms of poisoning may start to develop anywhere from a couple weeks to several months from when the plant was first ingested by the horse. The toxins damage the lung tissue, resulting in permanent damage to the respiratory system. Horses tend to like the taste of A. riparia
, so it is important to be diligent about removing this plant from pastures where horses might graze. A. riparia
is most toxic when it is in full bloom.