Alligator weed: declared pest

Page last updated: Monday, 20 July 2020 - 2:52pm

Please note: This content may be out of date and is currently under review.

Alligator weed (Alternanthera philoxeroides) 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: present in WA

Alligator weed is a perennial aquatic plant that can also grow on swampy land and in irrigated domestic gardens. On water, it forms dense floating or rooted mats up to one metre deep. It prefers static or slowly flowing fresh water with high nutrient levels, but will withstand brackish conditions.

Alligator weed spreads by stolons that grow outwards, forming new roots at the nodes. Short lengths of stem can break off and develop into a new plant. It forms seeds, but they do not appear to be viable in Australia, therefore spread is entirely by vegetative reproduction. It is a Weed of National Significance.


Stem: Hollow with hairy grooves on opposite sides of the internodes.

Leaves: Dark green, opposite, joined directly to the stem (ie no leaf stalks) two to seven centimetres long, 5-40 millimetres wide. Blade - waxy surface and conspicuous veins radiating from midrib

Flowers: Silvery white in spherical or cylindrical heads 1.2-1.4 centimetres diameter, borne on hairless or only slightly hairy stalks two to seven centimetres long arising from the leaf axils. Five petals five to six millimetres long, four-nerved and papery when dry.

Fruit: Brownish, thin skinned - bladder like and flattened.

Seed: Not quite round, and smooth surface, rarely, if ever produced in Australia.

Agricultural and economic impact

An environmental water weed. Toxic to livestock if eaten.

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). Use the links on this page to reach alligator weed in WAOL.

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.

Search > detect > report

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

Control method

Control methods for this declared plant can be found through the alligator weed control link.

Contact information

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