Ceratophyllum demersum



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

Other Common name: Coontail


Hornwort is a submerged freshwater weed found in still and flowing waters of streams, rivers, lakes and ponds. It has been found 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 is an unwanted organism under the Biosecurity Act 1993, and is banned from sale, propagation, and distribution under the National Plant Pest Accord.


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.

Eradication from the South Island

Hornwort was initially found in Moutere Stream and some private ponds in Motueka. All infestations were eradicated.

Hornwort was identified in Centennial Park Lake in Timaru 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).

A survey in December 2008 to monitor the effect of this control work revealed no visible signs of the weed. MPI revoked the Restricted Place notice for Centennial Park Lake and the lake was returned to normal use by members of the public. Centennial Park Lake was monitored by MPI for five years. No hornwort was detected and hornwort was declared eradicated from the lake in 2013. 

What to do

If you find hornwort in the North Island please 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 MPI on the Exotic Pest Hotline 0800 80 99 66.

Page last updated: 13 October 2014