Also known as
Oxygen weed, lakeweed, Elodea densa
Where is it originally from?
What does it look like?
Submerged, bottom-rooting perennial (<5 m). Slender, brittle, buoyant stems (3 mm diameter) are much-branched. Linear, dark green leaves (15-30 x 4 mm) are in whorls of 4-6 (occasionally 3 near base). From November to January it produces flowers (20 mm diameter) that are white, 3-petalled with yellow stamens, and that sit on the surface of the water. As only male plants are found in New Zealand, no seed is set.
Are there any similar species?
Lagarosiphon, Elodea, Hydrilla are all similar.
Why is it weedy?
Grows in most still or slow-moving, highly lit submerged sites, and tolerates a wide range of temperatures. Stems break and fragments root downstream or wherever they are dumped. Grows rapidly and forms dense patches, is long-lived, and tall enough to overtop smaller native species; it lacks native plant competitors of similar height.
How does it spread?
Loose stem fragments root at any node, colonising new sites. Water flow spreads it within catchments, and new catchments are infested by fragments spread by boats and trailers (occasionally motor cooling water), eel nets, diggers, people liberating fish, and floods from ornamental ponds. Birdspread is not a factor.
What damage does it do?
Forms vast underwater 'meadows', shades out smaller native species, and prevents seedlings of native species establishing. Large clumps can dislodge from the underwater 'meadows', causing flooding. Rotting vegetation stagnates water, killing fauna and flora.
Which habitats is it likely to invade?
Rivers, lakes, dune lakes, and other waterbodies with moderate to high light and temperatures in the range of 10-25 degrees C.
What can I do to get rid of it?
In small ponds, this plant can be killed by first using mechanical clearance, taking care not to spread fragments, followed by bottom lining of the pond. For larger infestations or infestations in flowing water, contact your regional council or local Department of Conservation office for advice.