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Book cover - Common Weeds of New ZealandAn Illustrated Guide to
Common Weeds
of New Zealand


Ceratophyllum demersum


Ceratophyllum demersum - hornwortReproduced from
Common Weeds of New Zealand
by Ian Popay, Paul Champion & Trevor James
ISBN 0 473 09760 5
by kind permission of the
New Zealand Plant Protection Society

Publication or other use of images or descriptive text on these pages is unauthorised unless written permission is obtained from the authors and publisher.
Appropriate acknowledgement of the publication Common Weeds of New Zealand must always be given.

Available from Touchwood Books

Wholly-submerged, free-floating or lightly anchored, rootless, delicate aquatic perennial. Has forked, finely divided leaves. Forms very dense under-water masses of vegetation in freshwater up to 10 m deep.

  • Flowers Green (female), whitish (male), no petals, minute. Solitary in leaf axils, stalkless, with ten to twenty stamens, adapted for underwater pollination.
  • Fruit Black oval nuts 5 mm long, with one terminal and two basal spines.
  • Leaves Dark green, up to 4 cm long, in whorls of seven to twelve densely crowded at the stem apex and increasingly spaced down stems. Leaves equally forked once or twice into rather stiff, tapering segments.
  • Stems Floating, submerged, up to 1.5 m long, branched, stiff and rather brittle. Readily broken by waves, current or boats to float away and establish in shallow water.
  • Roots No roots, lightly anchored in sediment by buried leaves and stems.


Still or slow-flowing freshwater of lakes, lagoons, ponds, rivers and drains.


Locally abundant in the Waikato River system including hydro lakes. Common in Auckland, Waikato, Rotorua Lakes, Taupo, Hawkes Bay and Wellington in NI. Found throughout the world.


May be confused with parrot's feather (Myriophyllum aquaticum) which has pinnate, rather than forked leaves. Listed on the National Pest Plant Accord (see Introduction for details).

Derivation of botanical name

Ceratophyllum (Gr.) = horned leaved, the leaves resembling antlers; demersum (Lat.) = submerged.

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Last updated: July 13, 2014