Also known as
Where is it originally from?
Europe and Asia
What does it look like?
Erect, hairy, summer-green perennial herb (<1-2 m, occasionally <3 m) with a taproot and fibrous roots that form dense surface mats and produce up to 50 stems per rootstock. Much-branched stems are 4-8 sided, pink at base and die off in winter. Narrow leaves (20-100 x 5-25 mm) are usually paired. Showy, densely hairy flowerhead spikes (20-25 cm long, Dec-Feb) are made up of purple-magenta flowers with 5-6 petals which are followed by blackish seed capsules (3-5 mm long).
Why is it weedy?
Rapidly invades damp ground and shallow water. Overtops native species with dense bushy growth, is long-lived and produces millions of long-lived, highly viable seeds from an early age. Tolerates hot or cold conditions and low to high nutrient levels in the water, but is intolerant of salt water.
How does it spread?
Seed is spread by the movement of water and contaminated machinery, soil, livestock and hay.
What damage does it do?
Forms massive, tall, impenetrable stands, excluding all other species. Destroys wetland and marginal habitats and food sources for many fish and bird species, and causes blockages and flooding.
Which habitats is it likely to invade?
Invades wetlands, lakesides, streams, swamps, bogs that are damp in winter and drier in summer, and creeps onto dry margins.
What can I do to get rid of it?
Start control work at margins to prevent further seeding, and work downstream where possible.
1. Weedmat: kills whole patches. Start at top of infestation, leave 3-4 months.
2. Spray: glyphosate (10ml/L). Respray until eliminated.
3. Spray terrestrial sites: triclopyr 600 EC (30ml/10L) or triclopyr 120g/L (15ml/L). Follow up 3-monthly.
4. Lower water level if possible and mechanically remove.
5. Raise water level if possible for 2-3 weeks to drown the weed.
What can I do to stop it coming back?
Plant tall trees adjacent to infested margins to shade out weed, and deepen water if possible to prevent growth.