Also known as
Mistweed, river eupatorium, Eupatorium riparium
Where is it originally from?
What does it look like?
Erect or sprawling many-stemmed herb or subshrub (<0.5-1.5 m tall) with a perennial fibrous rootstock. Stems are usually purple and are covered in purple-striped non-sticky hairs, become woody with age, have branches in opposite pairs, and occasionally die back in winter. Willow-like leaves (60-100 x 15-25 mm) are in opposite pairs and are coarsely serrated except near their base. Clusters of small white flowers (4-5 mm diameter, Aug-Jan) are followed by black 5-angled seeds (2 mm long).
Are there any similar species?
Mexican devil (A. adenophora) is similar.
Why is it weedy?
Grows very densely and overtops groundcovers, is long-lived, quick-maturing, and produces a large number of highly viable, well-dispersed (probably short-lived) seed. Drooping stems can also take root in wet sites wherever they touch the ground. Tolerates deep shade and damp, damage and grazing, salt, and most soil types, but is limited by dry conditions and frost. Poisonous, so usually shunned by livestock.
How does it spread?
Seed is spread long distances by wind and is also spread by water. Seed sources include roadsides, rivers, quarries, plantation forest, and reverting pasture.
What damage does it do?
Forms very dense colonies, preventing the seedlings of native species from establishing in a wide range of habitats, especially riparian areas. Invades strips along waterbodies, replacing vulnerable species, causes sediment buildup in streams, impeding flow, degrading habitats, and causing flooding, and can also cause instability on steep sites.
Which habitats is it likely to invade?
Damp forest and margins, intact or disturbed bush, light gaps, shrublands, wetlands, streamsides, river systems, inshore and offshore islands, slips, alluvial flats, coastal and estuarine areas, and occasionally places where low growing epiphytes would usually grow.
What can I do to get rid of it?
1. Dig or pull out small infestations (summer): Expose roots.
2. Spray (spring-summer): glyphosate (20ml/L + penetrant).
3. Spray (spring-summer): metsulfuron-methyl 600g/kg (20g/100 L (spraygun) or 3g/10L (knapsack)). Add penetrant in winter. Spray lightly, not to run off. Avoid water contamination.
What can I do to stop it coming back?
Pulled plants resprout if roots (occasionally even stems) touch damp soil and seed bank reinfests bared ground. There is a biological control, the Entyloma fungus, available - check with your regional council to see if this is suitable for your site.