Help us hunt down hornwort
Environment Canterbury media release: 7th May 2025
Do you have an aquarium, pond, or stream on your property? We’re currently on the lookout for the invasive aquatic plant hornwort (Ceratophyllum demersum) after reports of it being bought and sold online in Canterbury.
Hornwort is often purchased as a plant for aquariums in New Zealand — but when it escapes outdoors, the effects can be devastating.
Hornwort has earnt the title of ‘New Zealand’s worst submerged weed’. Its rapid growth can completely dominate local waterways at an alarming rate. It outcompetes native species down to 15 metres in depth, decimating native lake plants. Large infestations can also affect water flow and block key hydro-electrical infrastructure. In areas where hornwort has taken over, recreational activities such as fishing, swimming, and boating have become severely limited and, in some cases, near impossible.
What to look out for
- Feathery, dark green leaves in whorls (rings) around the stem.
- Leaves are rough to touch and densely crowded at the tip and sparse at the base.
- No roots, so it can float on the water.
- In waterways, it can form dense mats at or just below the surface.
The great hunt for hornwort
You're not in trouble if you think you might have hornwort in a home aquarium or pond! In fact, you will be helping us. Take a picture, record the location, and report sightings to us:
If it turns out to be hornwort, we will organise its removal and you’ll go in the draw to win a prize pack to put towards your pond or aquarium.
Hornwort was successfully eradicated from the South Island back in 2013. With these latest reports, it’s important we all do our part to protect Canterbury’s waterways for the future.