Unwanted Organism, National Interest Pest Response (South Island only), National Pest Plant Accord
- Scientific Name: Ceratophyllum demersum
- Origin: Introduced plant. Native to temperate regions eg., USA
- Type: Weed
- Plant Life-form Type: Submerged
- Taxonomy: L.
- Presence: Widely naturalised in North Island, all South Island sites have been targeted for eradication with no plants seen since 2008, first record from Napier 1961
- Habitat: Moderate flowing to still water bodies, growing to >15 m deep in some clear water lakes
- Temperature (°C): May-20
- pH range: 6.0 -9.0
- Hardness (ppm): 30 - 300
- Light: Low - Very High
- Height: 50 - 2000mm+
- Difficulty: Easy
- Notes: Currently New Zealand's worst submerged weed, affecting indigenous plant biodiversity, hydro-generation, irrigation, flood protection and recreation. A submersed plant of 1 - 2 metres in length. Has long rope-like brittle fronds. Very popular with coldwater enthusiasts. Grows lank and sparse in tropical tanks.Common in the Waikato river
- Propagation: Propagation by fragmentation of its brittle stems. Flowers occur on this species, but there is no evidence of seed production in New Zealand
- Features: Leaves are whorled and 10-40mm long. The leaves fork once or more into linear segments which are toothed (the teeth look like tiny horns ? hence the name hornwort, wort being old English for plant). Flowers are minute and located at the base of the leaves. Hornwort has no roots, instead its lower leaves anchor it to sediment. It can survive as a free floating mat absorbing all the nutrients it needs from the surrounding water.
- Source: TIFBIS
- Regional: Regional Pest Management Strategy: BOP, NSN/TAS, STL, MWT, WGN, AUK, HKB.