Weed Identification

Australia > > Alligator Weed

Alligator Weed

Alternanthera philoxeroides

THIS PLANT HAS BEEN DECLARED A WEED OF NATIONAL SIGNIFICANCE

Family: Amaranthaceae.

Form: Water plant

Origin: Native to South America.

Flowers/Seedhead: Flowerheads: White, cylindrical to globe-shaped heads that are papery to touch.

Description: Perennial with mostly hairless surface stems that root at the nodes (stoloniferous) and underground stems producing shoots & roots (rhizomatous). Can form dense mats in or out of water.

Distinguishing features: Distinguished by white flowerheads on stalks to 9 cm long that arise from the leaf-stem junction, opposite leaves and hollow stems especially when growing in water.

Dispersal: Viable seed not recorded in Australia. Spread by pieces in mud attached to machinery, in turf and by flood. Breakup of stems and further spread is increased by herbicide use and the plant's reaction to the damage caused by the flea beetle, Agasicles hygrophila.

Confused With: Other species of Alternanthera or species of Water Primrose, Ludwigia spp. or Smartweeds, Persicaria spp. Difficult to identify in dense vegetation and when not in flower.


Alligator Weed showing stalk to
flowerhead

Notes: Amongst the worst aquatic weed threats in Australia. Adapted to growing on damp land, occasionally flooded land, in shallow water (rooted in the substrate), attached to the bank (in deep water) or free floating. Will survive for a few days in sea strength salinity and thrive in 10% sea strength (3,500 mg/I) saline water. Introduced to Newcastle NSW in 1940s, it has spread to all States, aided by its misguided use as a culinary herb.

References:

    Noxious Weeds of Australia. W. Parsons and E. Cuthbertson, 1992, pages 154–157. Biology of Australian Weeds. R. Groves et al. (eds), Vol. 1, 1995, pages 1–12.

Web References: Search Australian web sites for further information on this weed.

Alternanthera sessilis, clustered flowerheads in leaf axils
Alternanthera
denticulata
Hollow purplish stem
of Alligator Weed

This weed has been included in the WEEDeck field guide as card W01

More information about WEEDeck is available from Sainty & Associates Pty. Ltd.

 

Australia > > Alligator Weed

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