Hydrocharitaceae (frogbit) family
Also known as
Canadian pondweed, oxygen weed
Where is it originally from?
Central and South America
What does it look like?
Submerged, bottom-rooting perennial, growing to 8+ m. Brittle, branched, slender stems (1 mm diameter). Translucent, linear, dark green leaves (6-12 x 2 mm) are in whorls of three (opposite at base). Male (very rare) and female flowers are on separate plants. Produces white, 5-petalled flowers (5 mm diameter) tinged with purple that sit on the surface of the water on long thread-like stalks. No seed is set in New Zealand.
Are there any similar species?
Lagarosiphon, Egeria, Hydrilla are all similar.
Why is it weedy?
Grows in moderate to highly-lit submerged sites and tolerates a wide range of temperatures. Moderate growth rate, tall, long-lived, dense, and crowds out and overtops smaller native species. Brittle stems break and fragments root downstream or wherever they are dumped. Lacks native plant competitors of similar height, but unlike other oxygen weeds, often co-exists with native species.
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, and people liberating fish. Birds do not spread it.
What damage does it do?
Can form deep underwater 'meadows', shading out smaller native species and prevents seedlings of native species establishing. Large clumps dislodge from these 'meadows', causing flooding. Rotting vegetation stagnates water, killing fauna and flora. Tends to be replaced by Lagarosiphon or Egeria and other weeds if these are also introduced into the same waterbody.
Which habitats is it likely to invade?
Rivers, lakes, dune lakes, and other waterbodies with moderate to high light and temperatures under 28 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.