Canadian Pondweed

North America

General description

Submerged, bottom-rooting perennial, to 8+ m. Stems slender, brittle, branched, 1 mm diam. Leaves in whorls of 3 (opposite at base), linear, 6-12 x 2 mm, translucent dark green. Male (very rare) and female flowers on separate plants. Flowers on surface, on long thread-like stalks, 5-petalled, 5 mm diam, white, tinged purple. No seed set in NZ.

All rivers, lakes, dunelakes, other waterbodies with mod-high light and temperatures under 28°C.

Stem fragments root at any node, colonising new sites. New catchments invaded by contaminated boats and trailers, eel nets, diggers, people ‘liberating’ fish.

Can form deep meadows, shades out smaller native species, prevents recruitment. Large clumps dislodge, causing flooding. Rotting vegetation stagnates water, killing fauna and flora. Threat to dune lakes. Tends to be replaced by lagarosiphon or egeria and others if introduced into the same waterbody.

Site management

Almost impossible to fully eradicate. In narrow waterbodies manipulate the plant’s growing conditions by reducing light, making it less competitive, eg plant trees to create shade.

Recommended approaches

1. Bottom lining: kills whole patches including roots. Lay carefully to ensure fragments are not released.
2. Lower water level, mechanically remove, use bottom lining.
3. Suction removal. Mulch. Generally not recommended in large still waterbodies as high chance of fragments spreading. If used, work methodically to trap all fragments, ensure collection bags do not leak. All sites below top of infestation are always at risk.
4. Grass carp: requires consent, contact DoC.

Caution: when using any herbicide or pesticide PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.

Similar species

Egeria densa

Egeria usually has leaves in whorls of 4 around stem. Canadian pondweed has whorls of 3.

Lagarosiphon major

Lagarosiphon has leaves that curl downwards and are arranged in opposite pairs.

Search tags

RPMS status

Not a legally declared pest plant
Canadian pondweed - Main species image