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K’gari Weed Spotters: Watch Out For Alligator Weed

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Native to South America, alligator weed Alternanthera philoxeroides is a perennial plant that grows on land in damp soil or water as dense floating mats.

Alligator weed’s flower and leaves (Photo: DAF)

Alligator weed affects water flow, water quality, native plants and native animals and has significant economic and social impacts.

Alligator weed poses an extreme threat to waterways, wetlands and irrigated croplands from Cape York to Queensland’s southern border. The weed has been reported both in Bundaberg and Sunshine Coast.

The plant is a vigorous weed that grows in water and on land. It has hollow stems, and the leaves are dark green with a distinct midrib, 2–12cm long, 0.5–4cm wide, arranged in opposite pairs along the stem. There is no leaf stalk. Flowers are white, paper-like, and ball-shaped on short stalks 2–7cm.

Alligator weed leaf (Photo: DAF)

The plant’s ideal habitat is in fresh water with a high nutrient level, although it can tolerate brackish water. It can establish in semi-aquatic areas, wetlands, stream and creek banks, and on land.

The plant forms new shoots in spring from nodes on existing stems. Although it does not produce viable seeds, regrowth occurs quickly from stems or underground rhizomes. Flowers appear around mid-summer. The plant can be spread by stems breaking and floating away to form new mats (especially after floods) or by cuttings in soil or equipment.

Alligator weed poses an extreme threat to waterways and wetlands. It restricts water flow in creeks, reduces water quality, water bird and fish activity, kills fish and submerged native plants and replaces native wetland plants. 

Alligator weed infestation (Photo: DAF)

Successful mechanical removal of alligator weed is complicated since the plant can re-establish itself from very small pieces of stem. In addition, alligator weed grows in different situations, each requiring particular herbicide controls. For more information, please read the alligator weed fact sheet.

Three biological control agents from South America have been introduced into NSW. Two of these insects, the flea beetle (Agasicles hygrophila) and stem-boring moth (Arcola malloi), are established and contribute to the control of alligator weed growing in water but not on land.

Please keep your eyes open – we don’t want alligator weed on K’gari!

Article adapted from Weed Spotters Network Queensland, Department of Agriculture and Fisheries

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