Oxygen weed
Hydrocharitaceae - Egeria densa

What does it look like?

Bottom-rooted submerged perennial aquatic herb.  Stems can be more than 3m in length.  Leaves are up to 5 x 30mm, in whorls of four to five.  Flowers are white, approximately 20mm in diameter, borne at the water’s surface from November-January.  All New Zealand plants are male, so no seed is set.

Still to moderately flowing water bodies to a depth of about 7-8m.  It prefers high nutrient water bodies and silty or sandy substrates.  Optimum water temperatures, 10-25°C.


Why is it a problem?

History of invasiveness overseas.  History of displacing native species as well as the oxygen weeds Canadian pondweed and lagarosiphon (especially in warmer waters).  Tolerates low light levels associated with turbidity.  Less competitive in low nutrient water bodies.

Moved between water bodies by humans through deliberate releases as well as accidentally on machinery and fishing equipment.  Public accessibility of site strongly predicts invasion likelihood.  Spreads within catchments via natural water movement.

Control Methods

Freshwater weeds are difficult to eradicate once established, but are possible to control. Before you start thoroughly 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.

Small infestations

  • Physical control: 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.

Large infestations

  • Herbicide: 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.

  • 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.


Related Links