Ceratophyllum demersum



Legal Status: Unwanted Organism - DOC
Status in New Zealand: Eradicated
Organism: Marine and freshwater plants

Other Common name: Coontail


Hornwort is a submerged freshwater weed found in still and flowing waters, growing to depths of 16 m in clear deep lakes. Leaves are finely divided, with minute teeth which make the plant feel rough to the touch. It lacks roots but has modified leaves that anchor the plant in bottom sediments.


New plants can form from each piece of the easily broken stems. 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.

Where is it found?

Hornwort is currently widely established in the North Island. In the South Island, it has been found in the Moutere Stream and some private ponds in Motueka where it was thought to be eradicated. In February 2006, a small population was discovered in Centennial Park Lake in Timaru.

MAFBNZ aims to eradicate and exclude hornwort from the South Island as part of its National Interest Pests Response programme. It is an unwanted organism under the Biosecurity Act 1993 and is banned from sale, propagation, and distribution.

Centennial Park Lake incursion

Hornwort was identified in Centennial Park Lake and the north branch of the Otipua stream in February 2006. To enable control work to take place and to ensure the weed did not spread, a Restricted Place notice was put in place soon after, closing the lake and affected stream areas to all use (including paddling, fishing, kayaking and swimming).

The latest survey in December 2008 to monitor the effect of this control work has revealed no visible signs of the weed.

As no signs of hornwort were found in the latest survey, MAFBNZ has revoked the Restricted Place notice for Centennial Park Lake. This means the lake can return to normal use by members of the public.

Centennial Lake will continue to be monitored for the next five years before hornwort can be declared eradicated from the lake.

What to do

Hornwort is still of limited distribution in much of New Zealand, and was only discovered in the South Island in 2006. In the North Island contact your Regional Council to determine the status of this species and responsibility for control and/or advice on control. Report all sightings in the South Island to MAF Biosecurity New Zealand on 0800 80 99 66.

Page last updated: 18 June 2013