Also known as
Florida elodea, water thyme
Where is it originally from?
Europe, Africa, India, South East Asia, Australia
What does it look like?
Submerged, bottom-rooting perennial (<9 m), with slender, brittle and branched stems (1 mm diameter). Translucent, dark green leaves (6-12 x 2 mm) in whorls of 3-8 have distinctly toothed margins, and the plant also produces small tubers and turions (hibernating buds). Only male flowers are present in New Zealand, so no seed is set.
Are there any similar species?
Lagarosiphon, Egeria, Elodea are all similar.
Why is it weedy?
Grows in moderate to highly-lit submerged sites with low to high temperatures. Moderate growth rate, height, long life and density, and overtops smaller native species. Brittle stems break, and fragments and turions root downstream or wherever they are dumped. Lacks native plant competitors of similar height.
How does it spread?
Loose stem fragments root at any node and turions also form new plants. Spread within catchments by flowing water, and new catchments are infested by fragments spread by boats and trailers, (occasionally motor cooling water), eel nets, livestock, and diggers. Birds are unlikely to spread it.
What damage does it do?
Forms vast deep underwater 'meadows', shading out smaller native species and preventing seedlings of native species from establishing. Large clumps dislodge from the 'meadows', causing flooding. Rotting vegetation stagnates the water, killing fauna and flora.
Which habitats is it likely to invade?
Rivers, lakes, dunelakes, and other waterbodies with moderate to high light levels.
What can I do to get rid of it?
If you suspect that you have found hydrilla, do not touch it. This pest plant is under an active national eradication programme, and any attempt to get rid of it could accidentally spread the pest. Any plants found that are suspected to be this species should be reported to Biosecurity New Zealand using the pests-and-diseases hotline on 0800 80 99 66. They will work with you to control the weed.