Myriophyllaceae (milfoil) family
Also known as
Brazilian milfoil, water milfoil, Myriophyllum brasiliense
Where is it originally from?
What does it look like?
Bottom-rooted, perennial floating and emergent plant with stolons, fibrous roots, and stems (5 mm diameter) that grow to 2 m long (3-4 m in flowing water) emerging 10 cm above water and rooting at lower nodes, with submerged parts become bare. Feather-like blue-green leaves (25-45 x 7-15 mm) are in whorls of 5-6, and are each divided into 25-30 leaflets (7 mm long). From September to February, minute female flowers are produced, but no seed is set in New Zealand.
Are there any similar species?
Hornwort (Ceratophyllum demersum) is similar. Five native Myriophyllum species all have stems less than 1 m long (except the endangered M. robustum), but are not blueish and are not weedy.
Why is it weedy?
All nodes can take root and it forms mats in still or slow water or on damp ground. Prefers water that has high nutrient levels, high sediment levels (especially peat levels) and which is polluted, but tolerates damage, grazing, hot and cold temperatures and salt water at low levels.
How does it spread?
Within catchments it is spread by flowing water, and new catchments are infested by fragments spread by boats and trailers, eel nets, diggers, and people 'liberating' fish. Birds are unlikely to spread it.
What damage does it do?
Forms dense mats, shading out existing native species and preventing new seedlings of native species from establishing, and replaces species that usually grow on the margins of waterbodies. Large clumps dislodge, causing flooding, and rotting vegetation stagnates water, killing fauna and flora.
Which habitats is it likely to invade?
Disturbed, polluted, high nutrient, well-lit, still or slow-moving waterbodies, as well as wetlands, water margins, streams, rivers, slightly saline estuary edges and river mouths.
What can I do to get rid of it?
Before you carry out any control work, make sure the plant you are targeting is not a valued native species.
1. Rake up (November-January): Leave on site to rot down. away from water, and then follow up from February to April by spraying any regrowth.
2. Weedmat: Lay carefully to ensure fragments are not released, start at top of infestation, leave 3-4 months.
3. Spray terrestrial sites (spring-autumn): glyphosate (20ml/L + penetrant) spray 4 times over a 10 or more week programme.
4. Lower water level, mechanically remove, use weedmat to cover the area or dry out thoroughly for 2-3 weeks.
What can I do to stop it coming back?
Plant trees adjacent to narrow waterbodies to create shade, create riparian strips, and remove pollution sources to prevent nutrient runoff.