Alligator Weed (Alternanthera philoxeroides)

Provided by Dow AgroSciences.

Alligator weed infestation and close-up of stem.

Alligator Weed (Alternanthera philoxeroides) is a perennial with reddish, soft hollow stems with nodes 50-100mm apart. Stems can extend many metres into water and up to a metre above the waters surface. Alligator weed is a strongly rhizomatous plant with roots extending downwards, and growing shoots extending upwards, from nodes. Shoots branch and re-branch to form dense, well rooted mats that float on water. Leaves are waxy, elliptical in shape, 5-10 cm long and grow in opposed pairs, generally without stalks. White flowers form solitary, clover-like heads up to 13mm in diameter on short stalks at the end of stems.

Close-up of foliage and flower.

Alligator weed grows strongly in wet soil or in water in streams, river beds and ponds. Fresh and brackish water are tolerated.

Alligator weed was originally introduced into New Zealand from ballast in ships and is currently found in the Auckland and Waikato areas.

How to control Alligator Weed

Alligator weed is difficult to control when growing in water. In pasture, Tordon Brushkiller XT is registered as a broadcast and spot spray treatment, and will provide control without damage to pasture grasses. Clovers will be removed from the sward for approx 12 months. Repeat treatments may be required for complete control.

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