Also known as:
Oxygen Weed
South America

General description

Submerged, perennial, freshwater, usually bottom rooted, long slender, much branched leafy stems <5m long; can also form free-floating mats. Denser and more leafy than other oxygen weeds, whorls of 4-5 leaves. Numerous white flowers <20mm just above water surface in summer.

Static or flowing enriched freshwater: dams, ponds, drains. Prefers sandy, silty sediments, moderate to high light levels, 10-25ºC.

Male plants only in NZ, no viable seed. Stems fragment and root easily; spread locally, carried to other waterways by boats, trailers, fishing gear and machinery.

Dense vegetation blocks waterways impeding drainage, disrupting recreational activities choking hydro turbines. Out-competes native species. Shades out smaller plants.

Site management

Almost impossible to kill, except in small patches. In narrow waterways, reduce light with riparian planting.

Recommended approaches

1. In the Great Barrier Island area it is defined as Total Control, so if found there report it immediately to the Auckland Council Biosecurity team, who will arrange for its control at their expense.
2. Weedmat, leave 3 months.
3. Lower water level where possible, mechanically remove, use bottom lining.
4. Suction removal generally not recommended in large still waterbodies, high chance of fragments spreading. If used, trap all fragments, ensure collection bags do not leak. In moving water all sites below top of infestation are at risk; water can shift infestation boundary downstream.
5. Grass carp. Requires consent, refer to 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

Lagarosiphon major

Lagarosiphon leaves are not arranged in whorls (arranged in spirals) and curve downwards.

Hydrilla verticillata

Hydrilla has toothed leaf margins.

southern naiad
Najas guadalupensis

S naiad has toothed leaf margins and it's leaves are on short branches.

Search tags

RPMS status

Total Control - Great Barrier Island
Surveillance - Whole Region
National Pest Plant Accord species - nationwide
egeria - Main species image