What does it look like?
Hydrilla is a submerged, bottom-rooting perennial growing to 9m, with slender, brittle and branched stems (1 mm diameter). Translucent, dark green leaves (6-12 x 2 mm) in whorls of 3-8 that have distinctly toothed margins. The plant produces small tubers and turions (hibernating buds) that develop in bottom sediments and may be viable for up to 10 years in New Zealand. Only male flowers are present in New Zealand, so no seed is set.
Why is it a problem?
Hydrilla was first detected in lakes in Hawke’s Bay and has not been found anywhere else in New Zealand. The potential for hydrilla to establish in freshwater bodies in New Zealand is almost unlimited. All fresh waterways are at risk, including Lake Waikaremoana, Lake Taupō, the Rotorua lakes, and the South Island lakes and rivers.
Eradication of this plant is at a national level. A 2016 progress report stated that no plants had been found at the only known infestations in Hawke’s Bay for over five years, indicating that the species is well on its way to being eradicated from New Zealand.
Control methods
MPI has a goal of eradication by 2030.
An eradication programme is focused on the three Hawke’s Bay lakes infested with hydrilla. Activities also include checks of creeks, streams, and ponds in the surrounding area to ensure the weed has not spread to other water bodies.
The eradication programme relies on grass carp (Ctenopharyngodon idella) to eat the hydrilla plants and the herbicide Endothall to kill plants that the fish can’t access. Hydrilla growth is monitored for several years after last detections until we are confident no viable tubers remain. Flora and fauna surveys and water quality monitoring are regularly undertaken to check on the effect of grass carp on the lakes.
To prevent the spread of hydrilla, all potentially contaminated items (including boats, equipment, fishing gear and clothing) should be checked and all plant material removed before leaving the site.
Related links
- https://www.mpi.govt.nz/biosecurity/exotic-pests-and-diseases-in-new-zealand/active-biosecurity-responses-to-pests-and-diseases
- https://niwa.co.nz/sites/default/files/Freshwater%20invasive%20species%20of%20New%20Zealand%202020_1.pdf
- http://apps.mpi.govt.nz/applications/nzpests-view/Article/1004/Hydrilla
- https://www.mpi.govt.nz/news-and-resources/media-releases/significant-progress-made-towards-eradication-of-hydrilla-weed
- http://www.nzpcn.org.nz/flora_details.aspx?ID=4091
- https://www.mpi.govt.nz/biosecurity/long-term-biosecurity-management-programmes/national-interest-pest-responses-programme
- https://www.nzflora.info/factsheet/Taxon/Hydrilla-verticillata.html
- https://www.thisisus.nz/assets/Resources/New-Zealand-freshwater-weed-and-pest-visual-guide.pdf
- https://www.weedbusters.org.nz/what-are-weeds/weed-list/hydrilla
- NPPA and the control of weeds | NZ Government
Notify Council
If you think you've found this pest, please get in touch with our Biosecurity Team at biosecurity@hbrc.govt.nz or call us on 0800 108 838.
Management Programme
National Pest Plant Accord
All plants on MPI's National Pest Plant Accord (NPPA) are unwanted organisms under the Biosecurity Act 1993 whose formal or casual trade contributes significantly to their spread.
Plants on the Accord cannot legally be propagated, distributed, or sold in New Zealand. If allowed to spread further, these pest plants could seriously damage our economy and environment.
Notifiable Organism
Any organism specified under the Biosecurity Notifiable Organisms Order 2016 must be reported to MPI if spotted. If any of these organisms become established in New Zealand, they could severely affect our primary production, trade, and market access. If you spot a notifiable organism, you have a legal obligation under section 44 of the Biosecurity Act 1993 to tell MPI. Call the pests-and-diseases hotline on 0800 80 99 66.
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.
More informationNational Interest Pest Responses programme
Nine harmful weeds are managed under the National Interest Pest Responses (NIPR) programme. These pests could cause serious harm to New Zealand's environment and economy if they are allowed to spread.
More informationRules
Section 26ZM of the Conservation Act (1987)
In accordance with the Conservation Act 1987 section 26ZM, no person shall transfer live aquatic life or release live aquatic life into any freshwater. This includes native or introduced fish, plants, and invertebrates. The taking and holding of some organisms requires a special permit from MPI.
More informationTaxonomies
FamilyHydrocharitaceae
TypePlants
GroupSubmerged
HabitatFreshwater
Management ProgrammeNational Pest Plant AccordNotifiable OrganismUnwanted OrganismNational Interest Pest Responses programme
