Oxygen weed
Hydrocharitaceae - Lagarosiphon major

What does it look like?

Submerged, bottom-rooting perennial up to 5 m. Leaves (16 x 2 mm) have minute serrations along the edges, are arranged spirally around the stem, and are curved backwards or downwards. Tiny pinkish flowers are produced, but as only female plants are found in New Zealand, no seed is set.

Why is it a problem?

Grows rapidly in moderate to well-lit submerged sites ranging from low to high temperature, is tall, long-lived and dense, and overtops smaller native species. Lacks native plant competitors of similar height in New Zealand. Stems break easily, and fragments root downstream or wherever they are dumped.

Control Methods

Freshwater weeds are difficult to eradicate once established, but are possible to control. Before you start thooughly check the waterway's adjacent areas and inflows to see if the infestation has spread.

As most aquatic weeds grow from fragments, start control at the upstream end of the infestation. In narrow waterways you can reduce the growth of aquatic weeds with riparian planting to reduce light levels.

Physical control (small infestations)

  • Remove the weeds by digging and raking them up. Dispose of them on land so they dry up and die. Follow up regularly to remove growth.
  • Bottom lining (small ponds): If possible, lower the water level and cover the infestation with black polythene or weedmat (weighted down) for about three months.

Biological control

  • Grass carp can be used as a weed control agent in some cases. This requires a permit - contact the Department of Conservation for more information.

 Herbicide control (large infestations)

  • These are available for some freshwater weeds, and can be an effective control option. Check with your local council's biosecurity team before spraying, as spraying in or over water may require resource consent.

CAUTION: When using herbicide, PLEASE READ THE LABEL THOROUGHLY and follow ALL instructions and safety requierments.

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