Floating water chestnut: declared pest

Page last updated: Monday, 4 May 2020 - 10:17am

Floating water chestnut (Trapa species) is a declared pest in Western Australia (WA). This article describes the nature of the plant with links to requirements land owners/occupiers must adhere to and pest control methods.

Form: aquatic

Status: not present in WA

Floating water chestnut is native to Europe, Asia and tropical Africa.


Water chestnut is a rooted, floating plant that invades shallow to deep, fresh water habitats. Water chestnut can grow in 3.6–4.6 metres of water and forms dense, floating mats, often three layers deep.

Leaves: Leaves on the surface of the water are alternate, triangular in shape, strongly toothed and connected to the stem by an inflated petiole. Submerged leaves are feathery and either opposite or alternate.

Flowers: Flowering occurs from mid-summer to frost. Small, four petal flowers born in the leaf axils of younger leaves above the water give way to the nut-like fruit.

Fruit: The fruit have two to four 1.3 centimetre long, sharp, barbed spines. The spines can penetrate shoes.

Seed: The single-seeded mature fruit are woody and bear four sharply pointed horns.  When mature, the fruits fall from the plant and sink to the bottom of the water body. A seed dormancy period of four months has been found, overwintering at the bottom of the water body and germinating during and throughout much of the warm season to produce shoots that grow to the water surface, where the typical rosette is formed. Seed can remain viable for up to five years.

Agriculture and economic impact

An environmental aquatic weed. The dense, floating mats restrict light availability, reduce the oxygen content, displace other emergent and floating vegetation and affect the survival of native species and water quality.

Water chestnut mats also limits boating, fishing, swimming and other recreational activities.

Declared pest category

The Western Australian Organism List (WAOL) contains information on the area(s) in which this pest is declared and the control and keeping categories to which it has been assigned in Western Australia (WA). Search for floating water chestnut in WAOL using the scientific name Trapa.

Requirements for land owners/occupiers and other persons

Requirements for land owners/occupiers and other persons if this pest is found can be sourced through the declared plant requirements link.

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MyPestGuide™ Reporter
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Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080

Control method

Report the presence of this organism before undertaking control. Control methods for this declared plant will be provided after it has been reported.

Contact information

Pest and Disease Information Service (PaDIS)
+61 (0)8 9368 3080
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