Hornwort is an invasive freshwater weed that threatens most underwater plant communities, negatively affecting the environment and recreational values. It rapidly invades water with all different levels of clarity, temperature, light and nutrient level. Its thick growth crowds out native species, and blocks waterways. While the rotting vegetation stops the flow of water which kills fauna and flora. Hornwort has the potential to cause negative effects on environmental and recreational values in Otago.   

New plants form from stem fragments that break off, and through human activities on boats, trailers and equipment. 

Hornwort is found in several sites in the North Island of New Zealand. It is thought to have been successfully eradicated from the South Island and has not been reported in Otago. 

• Size:  Stems can grow up to 150 cm long. 
• Appearance: Hornwort is an underwater, free-floating or lightly-anchored plant. It has thin, dark green leaves in patterns of spirals of 7-12 and are densely crowded at the stem tip. They get more and more spaced out down the stem. Its stems are floating or submerged, stiff and easily broken. 
• Similar species: often mistaken for native Myriophyllum (water milfoil) species, however, these can be distinguished as Myriophyllum species have feathery not forked leaves, and their leaves are not stiff. 
• Flowers: In other countries, it produces tiny green or white flowers, but it is not known to flower in New Zealand. 
• Habitat: It grows in still or slow-moving waterbodies or lagoons in water up to 16m deep. It has the potential to establish throughout New Zealand. 

In the Otago Regional Pest Management Plan (2019-2029), Hornwort is in the exclusion programme to prevent Hornwort establishing in Otago to prevent negative effects on economic well-being and environmental values. 

Hornwort is declared an Unwanted Organism in the National Pest Plant Accord (NPPA). Any species declared a pest under the NPPA cannot be sold or be in a place where plants are being sold. Pest plants cannot be propagated, bred, multiplied, communicated, released, caused to be released, or otherwise spread.      

Otago Regional Council will be responsible for controlling the plant if it gets into Otago, most likely in collaboration with the Ministry for Primary Industries, Department of Conservation, Land Information New Zealand, and land occupiers. If you think you have spotted Hornwort, please let us know by calling 0800 474 082 or emailing biosecurity@orc.govt.nz 

Preventing the spread of Hornwort using the Check Clean Dry method is the best strategy, as removal of invasive freshwater pests is both difficult and expensive. 

Cleaning your craft and gear for biosecurity purposes is an important way to help protect the freshwater environment. Just as regular maintenance keeps your equipment in good condition, biosecurity cleaning keeps lakes and rivers healthy for everyone. 

To make sure you are not spreading any aquatic pests, the Check Clean Dry method is the best way to keep our waterways pest-free.