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


Alternanthera philoxeroides
alligator weed


Reproduced 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.

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Alternanthera philoxeroides - alligator weed

Aquatic perennial, forming dense floating mats on fresh or brackish water, and also creeping onto wet dune hollows and cropping land near waterways. Soft, hollow stems up to 2 m long. Opposite leaves 5-10 cm long. White flowers similar to white clover, but smaller.

  • Flowers White, small papery florets in six- to twenty-flowered, clover-like heads up to 13 mm in diameter. Flower heads solitary on stalks up to 9 cm long in leaf axils, or on short stalks at the ends of the branches. Flowers Dec-Feb.
  • Fruit Not produced in NZ and reproduction is only from stem fragments.
  • Leaves Waxy, elliptical to egg-shaped, 5-10 cm long, margins entire, in opposite pairs, stalkless or with a short leaf stalk. A ring of white hairs is found between the bases of the opposite leaves.
  • Stems Thick, soft, hollow, grooved, reddish, branching, to 10 m or more long, floating or creeping. Stem tips upright, as much as 1 m above the water.
  • Roots Fibrous, stems root from lower nodes.


Warm, shallow, enriched, fresh to slightly brackish water of drains, swamps, ponds, lagoons, stream banks, dune hollows.


NI only. Abundant in North Auckland waterways especially around Kaipara Harbour, Auckland City, Waikato River mouth and one or two sites in Waikato and Bay of Plenty. Originally from Brazil.


Likely to spread further south as a serious weed of waterways and of nearby crops. Insects introduced as biological control agents give good control in lakes, but not on wet land. Listed on the National Pest Plant Accord (see Introduction for details).

Related species

Nahui (Alternanthera sessilis), native to NZ, is much smaller, with stalkless axillary flower heads, occurring in damp open places, rarely in dry waste areas and gardens, locally throughout NI, and in the Lake Forsyth area in Canterbury.

Derivation of botanical name

Alternanthera (Lat.) = alternate anthers (the anthers are alternatively fertile and sterile); philoxeroides (derivation unknown).


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Web-notes: Weed Links

On this site

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External Links

WeedbustersWeedbusters New Zealand
Weedbusters is a weeds awareness and education programme that aims to protect New Zealand's environment from the increasing weed problem.
A free tool to assist farmers and agricultural professionals in decision-making regarding weed and pest identification, biology, impact and management.
Weed keyNew Zealand Weeds Key
An interactive identification key to the weeds of New Zealand. Developed at Landcare Research.

New Zealand Plant Conservation Network naturalised plants
Search for information on more than 2500 naturalised and weedy plants.
NZ Plant Protection SocietyNew Zealand Plant Protection Society
Their main objective: "To pool and exchange information on the biology of weeds, invertebrate and vertebrate pests, pathogens and beneficial organisms and methods for modifying their effects."
Massey UniversityMassey University Weeds Database
A site providing information about New Zealand weeds and weed control. It has a series of pages showing pictures of New Zealand weeds, notes on identification and control. It also provides information on a university paper entitled Controlling Weeds.

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Last updated: March 1, 2021