Asia, Africa, Australia, North America.

General description

Submerged, delicate, freshwater perennial; forms very dense underwater masses of floating vegetation to <10m deep. Dark green leaves <4cm long and equally forked into stiff tapering segments. Small separate male and female flowers (Nov-Mar) in each whorl. Stems floating, submerged, stiff, brittle. No true roots, can anchor lightly in sediment by burying leaves and stems.

Still or slow flowing freshwater: lakes, ponds, rivers, dams. Prefers fertile, slightly alkaline water, sandy/silty sediments. Low light tolerant. Sensitive to freezing and dehydration.

No viable seed in NZ. Stem fragments spread locally carried by water, boats, fishing gear, machinery.

Dense vegetation blocks waterways, impeding drainage, disrupting recreational activities, choking hydro turbines. Out-competes native species

Site management

Eradication almost impossible, control only possible where establishment is recent. Survey adjacent waterbodies thoroughly first. In narrow waterbodies, reduce light levels with riparian planting.

Recommended approaches

1. Weedmat for 18-28 days.
2. Lower water level where possible, use weedmat.
3. Suction removal generally not recommended in large still waterbodies as high chance of fragments spreading. If used, trap all fragments, ensure collection bags do not leak. In moving water all sites below top of infestation are at risk; water flow can shift infestation boundary downstream.
4. Moderately susceptible to diquat herbicide.
5. Grass carp: requires consent, refer to DOC.

Caution: when using any herbicide or pesticide PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed.

Similar species

parrots feather
Myriophyllum aquaticum

Parrot's feather has pinnate leaves as opposed to the forked leaves of hornwort.

Eurasian water milfoil
Myriophyllum spicatum

E. water milfoil has pinnate leaves as opposed to the forked leaves of hornwort.

Search tags

RPMS status

Surveillance - Whole Region
National Pest Plant Accord species - nationwide
hornwort - Main species image