Hornwort

What does it look like?

Hornwort is a perennial, submerged aquatic plant that does not form true roots. It may anchor itself to sediment via its stems or exist as free-floating mats. The branched stems can grow up to approximately 7 m in length. Leaves are narrow, branched, and arranged in whorls, measuring 10–40 mm long and forming a complex, bushy structure. Flowers are minute, and there is no evidence of viable seed production in New Zealand. New plants readily form from fragments of the easily broken stems.

Hornwort grows in freshwater environments such as drainage channels, lakes, and farm ponds. It prefers shallow shorelines and areas with sediment accumulation, making it less problematic in steep-sided reservoirs. It tolerates a wide range of nutrient conditions —from oligotrophic to hypertrophic lakes— and survives in temperatures ranging from 5°C to 30°C, with the ability to tolerate ice cover. It can occupy a variety of water depths, growing as deep as 16 m in clear, deep lakes.

Why is it a problem?

Hornwort rapidly invades water of varying clarity, temperature, light and nutrient level, and its dense growth habit crowds out native species. It is a major weed in hydroelectric dams, also impeding irrigation, drainage and other water uses.

Human movement is the main vector between water bodies. It may be spread accidentally, as fragments attach to nets, boats and other gear. It can also spread intentionally as habitat for co-released pest fish. Risk factors for lake infestation include proximity to densely populated areas, closeness to highways, and the size of the lake. Natural dispersal can also occur through downstream flow or flood-driven movement of plant fragments.

Control methods

Freshwater weeds are difficult to eradicate once established, but are possible to control. Before you start, complete a thorough check of the waterway's adjacent areas and inflows to see if the infestation has spread.
As most aquatic weeds grow from fragments, start control at the upstream end of the infestation. In narrow waterways you can reduce the growth of aquatic weeds with riparian planting to reduce light levels.

Physical control (small infestations)

• Remove the weeds by digging and raking them up. Dispose of them on land so they dry out and die. Follow up regularly to remove growth.
• Bottom lining (small ponds): If possible, lower the water level and cover the infestation with black polythene or weedmat (weighted down) for about three months.

Herbicide control (large infestations)

• Herbicide control is available for some freshwater weeds, and can be an effective control option. Contact us for advice, as spraying in or over water may require resource consent.
• CAUTION: When using herbicide, PLEASE READ THE LABEL THOROUGHLY and follow ALL instructions and safety requirements.

Biocontrol
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

• Check, Clean, Dry
• Freshwater Invasive Species of NZ 2020
• Ministry for Primary Industries
• New Zealand Plant Conservation Network
• NZ Flora
• NZ Freshwater Weed and Pest Visual Guide
• Unwanted Organisms Register
• Weedbusters
• NPPA and the control of weeds | NZ Government