What does it look like?
Lagarosiphon (oxygen weed) is a submerged, bottom-rooting aquatic perennial that grows 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?
It grows rapidly in moderate to well-lit submerged sites ranging from low to high temperature, forming vast deep underwater 'meadows' that shade out smaller native species, and prevent seedlings of native species from establishing. There are no native plant competitors of similar height in New Zealand. Stems break easily, and fragments root downstream or wherever they are dumped, causing flooding. Rotting vegetation turns water stagnant, killing fauna and flora.
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.
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.
Herbicide control (large infestations)
- Diquat is an effective herbicide for controlling Lagarosiphon, but there are regulations around its use as spraying in or over water may require resource consent, so please check with us first.
- CAUTION: When using herbicide, PLEASE READ THE LABEL THOROUGHLY and follow ALL instructions and safety requirements.
Biocontrol
- 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
Management Programme
National Pest Plant Accord (NPPA)
The NPPA is designed to prevent the sale, distribution and propagation of a set list of pest plants (the Accord list) within New Zealand. If allowed to spread further, these pest plants could seriously damage the New Zealand economy and environment.
More informationUnwanted Organism
An unwanted organism is any organism that's capable of causing harm to natural or physical resources (like forests and waterways) or human health. A number of introduced pests in New Zealand are classed as unwanted.
Taxonomies
FamilyHydrocharitaceae
TypePlants
GroupSubmerged
HabitatFreshwater
Management ProgrammeNational Pest Plant Accord (NPPA)Unwanted Organism
