Ceratophyllum demersum

Also known as: Coon's tail
Pest group: Freshwater | Plants
Pest type: Waterway plants

A submerged aquatic plant no taller than 7m that forms free-floating mats and displaces native vegetation and increases flooding risk.


  • Stems are branched and may anchor to sediment or form free-floating mats.
  • Leaves are long, narrow, branched and whorled, with complex architecture. Flowers are minute.
  • Vegetative spread from stem fragments, dispersed by water.
  • Human-mediated dispersal through contamination of boats, fishing gear and machinery and deliberate cultivation.
  • Habitats include freshwater bodies less than 15.5m deep and drainage ditches.

What you need to know

Forms dense stands and displaces native submerged vegetation down to 15m depth. Alters water flow, increases flooding risk, and impedes recreational access to water bodies. Likely to negatively impact crayfish/kōura.

Management approach

This is a declared pest managed under the Canterbury Regional Management Plan 2018 – 2038 (PDF file, 10.6MB) within the exclusion programme.


Pests in the exclusion programme are not known to be established in Waitaha/Canterbury. If these pests were to become widely established, their impacts could be severe. Therefore, early intervention by preventing their establishment is a cost-effective management approach.

Hornwort was declared successfully eradicated from the South Island in 2013 through the National Interest Pest Response programme led by the Ministry of Primary Industries.

The community should make us aware of any hornwort plants in Waitaha/Canterbury.

Hornwort is also an unwanted organism regulated under the National Plant Pest Accord.


Any species declared a pest 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.


Do not attempt to undertake control of hornwort yourself. Report any sightings to us.