Hornwort in stream

Hornwort (Ceratophyllum demersum) is also known as coontail, hornweed, lakeweed and oxygen weed. A native of North America, Europe, Asia and Australia this species was introduced to New Zealand in 1961.
Hornwort is a vigorous submerged freshwater perennial, which has the ability to form dense mats that smother other species. This plant does not produce roots, but the base of the stems are lightly anchored in sediment.

Hornwort has the ability to flourish in still or slow flowing water in depths up to 10 metres and produce stems up to 6 metres long.

Hornwort is most abundant from February to April. During the winter period, with cooler water temperatures, the plant dies down only to return once the water temperature rises.

With the ability to produce very prolific and dense growth, hornwort is difficult to eradicate once established. The leaves are needle-like with the lower leaves spaced along the stem in circles (whorls) of seven to 12. Towards the tip of the stem the leaves become more compact and the plant takes on the appearance of a miniature pine tree branch.

Hornwort forms dense stands preventing native regeneration. Hornwort also poses a threat to irrigation channels throughout the region.

Hornwort poses a serious environmental threat to the region's biodiversity and recreational areas.

	Hornwort

• Prevent the establishment of hornwort outside the containment zone.
• Manage hornwort within the containment zone so that it s not present in more than 30% of its potential habitat at any one time.

Hornwort is currently of limited distribution in areas in the Wellington region. With its ability to cause serious adverse impacts to native aquatic flora and impede recreational activities it is important to ensure this species is controlled.

There are control programmes for hornwort in all areas except the surrounds of Lake Wairarapa. Report infestations inside these zones to Greater Wellington. In the containment zone occupiers shall destroy all hornwort plants in waterbodies on land that they occupy.

You can see the zones on the map link in the box on the right of this page.

It is illegal to sell, propagate, or distribute any parts of hornwort in the Greater Wellington region.

Greater Wellington will undertake control of hornwort outside the containment zone. In the containment zone occupiers shall destroy all hornwort plants in waterbodies on land that they occupy.

Please contact Greater Wellington's Pest Plant Department for further information.

For more information or advice about pest plants, contact Greater Wellington's Biosecurity staff on 06 378 2484 or 04 526 5325 or email pest.plants@gw.govt.nz.