Alligator weed

Alligator weed is currently present in the ACT.

About the pest

Alligator weed (Alternanthera philoxeroides) is an invasive weed that grows in water and on land with the potential to seriously affect waterways, wetlands and floodplain areas.

Why it is a problem

Alligator weed has extremely vigorous growth and great tolerance of normal control measures. Key problems it causes include:

• Flooding hazards: In water the thick mats trap sediment and block stream flow, significantly increasing the risk and damage of floods.
• Agricultural damage: When it’s present in farmlands and pastures it ruins crops and contaminates soil.
• Recreational and health hazards: Weed mats in water make boating and swimming impossible.

What to look for

Alligator weed blooms over the spring and summer. It has:

• small white papery flower heads 8–10 mm in diameter, generally appearing from November to March
• leaves occurring in opposite pairs along the stems
• shiny, spear-shaped, sessile (no stalk) leaves about 2–7 cm long and 1–2 cm wide
• stems that grow to 60 cm high and up to 10 m long and have large, hollow internodes.

How it spreads

Alligator weed spreads rapidly through vegetative reproduction rather than by seeds. Even the tiniest broken stem fragment – as small as a single node with leaves – or a piece of root can grow into an entirely new, massive infestation.

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