Egeria

What does it look like?

Egeria is a bottom rooted, submerged perennial aquatic plant in the oxygen weed group. It’s a large, leafy species with stems that can grow more than three metres long. The leaves are 10–30 mm long and grow in whorls of four to five along the stem.

Male plants produce conspicuous white flowers about 20 mm in diameter, which appear at the water’s surface from November to January. All egeria plants in New Zealand are male, so the species does not produce seed here.

Egeria grows in still to moderately flowing water to depths of around seven to eight metres. It prefers nutrient rich water bodies with silty or sandy substrates, and grows best at water temperatures between 10–25°C.

Why is it a problem?

Egeria is highly invasive, displacing native aquatic plants and other oxygen weeds like Canadian pondweed and lagarosiphon, particularly in warmer waters. It grows rapidly in still or slow moving, well lit waterways and tolerates a wide range of temperatures and light conditions.

The plant spreads easily when stems break, with fragments rooting wherever they settle. It forms dense, long lived underwater beds that overtop smaller native species and has no native competitors of similar height. Although less competitive in low nutrient waters, it readily invades nutrient rich sites.

Egeria is spread mainly by human activity, including boats, trailers, fishing gear, machinery and floods, allowing fragments to move within and between catchments. Large mats can detach from established beds, blocking waterways, causing flooding, and creating stagnant conditions that reduce oxygen levels and kill aquatic plants and animals.

Control methods

• Freshwater weeds are difficult to eradicate once established, but they can be controlled. Before starting, check upstream areas, inflows and adjacent waterways to see how far the infestation has spread. Control should begin at the upstream end to prevent reinfestation.
• In small ponds, egeria can be controlled by careful mechanical removal, ensuring fragments are not spread, followed by bottom lining the pond. For large infestations or those in flowing water, contact us or Department of Conservation for advice.
• In narrow waterways, riparian planting can help reduce growth by limiting light. Small infestations may also be removed by digging or raking, with material disposed of on land to dry out and die. Where possible, lower water levels and cover infestations with weighted black polythene or weed mat for around three months.
• Herbicides can be effective for some freshwater weeds but spraying in or over water may require resource consent. Always check with our biosecurity team before using herbicides.
• CAUTION: When using herbicide, PLEASE READ THE LABEL THOROUGHLY and follow ALL instructions and safety requirements.

More information

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

• Visual guide to freshwater weeds and pests
• https://www.mpi.govt.nz/dmsdocument/3664-national-pest-plant-accord-manual-reprinted-in-february-2020-minor-amendments-only https://www.weedbusters.org.nz/what-are-weeds/weed-list/egeria/
• https://www.nzpcn.org.nz/flora/species/egeria-densa/
• https://www.nzpcn.org.nz/site/assets/files/0/18/289/egeria-densa.pdf
• https://www.doc.govt.nz/get-involved/apply-for-permits/all-permissions/interacting-with-freshwater-species/options-for-weed-control/grass-carp/