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 (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 informationNotifiable Organism
Some organisms must be notified to MPI if they're spotted in New Zealand. These include serious pests and diseases like foot-and-mouth disease, the rabies virus, fruit fly and disease-carrying mosquitoes. If a notifiable organism established in New Zealand, it could severely affect our primary production or 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.
The Biosecurity (Notifiable Organisms) Order 2016 lists notifiable organisms. New additions can be made by the Governor-General and MPI periodically reviews the list to ensure it is up-to-date.
Unwanted 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.
National Interest Pest Responses programme (NIPR)
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 informationTaxonomies
FamilyHydrocharitaceae
TypePlants
GroupSubmerged
HabitatFreshwater
Management ProgrammeNational Pest Plant Accord (NPPA)Notifiable OrganismUnwanted OrganismNational Interest Pest Responses programme (NIPR)
