Hydrilla is a submerged, rooted annual or perennial aquatic plant which grows to water depths of up to 9m. It forms very dense stands crowding out native aquatic plant species, and restricting light and depleting oxygen restricting.
Hydrilla stems vary in length from a few centimetres to several metres and are either creeping or erect. The leaves occur in opposite pairs, or typically in whorls of four, although they may range from three to eight and rarely 12 per whorl. Leaves are linear with a single spine cell at the end, and are usually about 12mm long and 2mm wide. Leaves are generally green, but often have small reddish-brown spots and stripes. The midrib is distinct and occasionally bears unicellular spines on the abaxial surface. The margins are strongly serrate, with fine translucent teeth that are visible to the naked eye.
In New Zealand only the dioecious male is present. Reproduction of New Zealand's hydrilla is solely vegetative. Apart from reproduction via stem fragments, hydrilla also produces specialised vegetative structures called turions and tubers. Turions, also called axilliary turions, are formed on erect stems in the water column. The mature turion is green, ovate and about 10mm in length, on small stem, covered with numerous overlapping pointed leaf scales.
Tubers, also known as subterranean turions, are swollen brown to white structures up to 15mm long, which develop in the bottom sediments. Turions remain viable for one to two years, while tubers have remained viable for four years overseas, and up to 10 years in New Zealand.
Where is it found?
Hydrilla is known from four lakes in the Hawke's Bay region: Lake Tutira and the adjacent Lake Waikopiro; Lake Opouahi; and Lake Eland. In the first three lakes, hydrilla forms significant weed beds. In Lake Eland, grass carp were introduced in 1988 and since 1990 only remnant plants were found. None have been found during annual surveillance in 2005, 2006, 2007 or 2008.
Hydrilla is a highly invasive water weed and is considered one of the world's worst. It's potential to establish in other fresh water bodies in New Zealand is almost unlimited. All fresh waterways are at risk, for example Lake Waikaremoana, Lake Taupo, and the Rotorua lakes, as well as South Island lakes and rivers.
If established in other lakes it can cause significant economic losses through blocking intakes and affecting power generation and irrigation. In shallow waterways it can restrict navigation, and impede water flow in irrigation and drainage channels.
The dense single species canopy formed by hydrilla displaces and excludes native vegetation particularly in the 1 to 5m water depth zone. The weed beds can also cause deoxygenation of the water with adverse impacts on animal life.
The weed beds can also become a nuisance for recreational users such as divers, bathers, anglers and boaties. There is also a risk of accidental drowning after becoming entangled in the weed. Plant material can also drift ashore in rafts, particularly after storms, where it rots and may become smelly. Recreational use may also be restricted as a result of controls designed to reduce the risk of spread.
Mahinga kai (traditional food gathering) is central to Maori culture; so that the losses associated with the presence of hydrilla has significant implications for Maori. Local iwi have concerns about the presence of hydrilla in the lakes as it presents a threat to the quality of the water bodies, a threat to other water bodies, prevents eeling and prevents the restoration of rongoa in the lake shallows.
Hydrilla is a national interest pest and is one of eleven pests established in New Zealand that the Ministry of Agriculture and Forestry Biosecurity New Zealand (MAFBNZ) has identified for either eradication or national control.
Hydrilla was first detected in Lakes Tutira and Waikopiro in 1963 and later found in Lakes Opouahi and Eland. It is has not been found anywhere else in New Zealand.
Hydrilla is one of the world’s worst submerged waterweeds. It out-competes other aquatic plants including natives and can remain dormant in sediments for up to ten years waiting for the right conditions to grow.
Weed beds of hydrilla are a nuisance to lake users such as bathers, anglers and boat users. Plant material washed ashore rots, reducing the aesthetic value of the lakes, and restricting access to water. It may also clog hydroelectric dams and block water intakes in water bodies where it is present.
Eradication is now feasible following the successful trial in Lake Eland, Hawke’s Bay, using grass carp (also known as white amur) to eat the plants, and the recent approval for the use in New Zealand of the herbicide Endothall by the Environmental Risk Management Authority.
Grass carp have been used successfully to manage water-weeds in other lakes in New Zealand.
Endothall has been used by MAFBNZ to successfully control the invasive aquatic weed hornwort in the South Island.
To eradicate Hydrilla verticillata from New Zealand by 2030.
The following documents are available.
- Hydrilla Lakes Flora and Fauna Survey and Urban Surveillance 2011 (3546 KB) MAFBNZ have developed a response to manage and eradicate the submerged weed hydrilla from the Hawkes Bay and hence from New Zealand.
- Organism Consequence Assessment (544 KB) MAFBNZ commissioned an organism consequence assessment of hydrilla from the National Institute of Water and Atmospheric Research (NIWA).
- Management Options Assessment (329 KB) Management options were developed for hydrilla by NIWA. The management options were considered by the Technical Advisory Group, which recommended that hydrilla should be eradicated.
- Assessment of Environmental Effects reports have been prepared for both endothall (7394 KB) and grass carp (1739 KB), by NIWA.The reports have been used in support of the regulatory approvals required under the Reserves Act 1977; the Conservation Act 1987; and, the Freshwater Fisheries Regulation 59.
- Hydrilla Lakes Baseline Supplement 1 (3656 KB) MAFBNZ commissioned a baseline survey of flora and fauna in hydrilla affected lakes in the Hawkes Bay from the National Institute of Water and Atmospheric Research (NIWA).
- Hydrilla Lakes Baseline Survey 2008 - Flora and Fauna (26137 KB) MAFBNZ commissioned National Institute of Water and Atmospheric Research (NIWA) to carry out a spring fish survey and vegetation surveys in hydrilla affected lakes in the Hawkes Bay, prior to and one month after, initial hydrilla control treatments commencing in December 2008.
- Hydrilla Lake Vegetation and Invertebrate Survey and Hydrilla Urban Site Surveillance 2009 (1610 KB)
- Hydrilla Frequently Asked Questions (354 KB)
- Fish population monitoring in lakes Tutira and Opouahi, Hawkes Bay: April 2011 (549 KB)
- Effects of weed removal by grass carp on the native fish populations in Lake Opouahi and Lake Tutira (939 KB)
For more information please contact
What to do
Propagation, spread and sale of hydrilla is prohibited under the Biosecurity Act 1993.
All sightings must be reported to Biosecurity New Zealand on 0800 80 99 66.
MAF contracts a monitoring and control programme aimed at eliminating hydrilla from New Zealand.
Page last updated: 15 March 2013