Alligator weed (Alternanthera philoxeroides) is regarded as one of the worst weeds in Australia because of its invasive tendencies, potential for spread, and economic and environmental impacts. It is an especially troublesome weed because it invades both land and water, and is very hard to control.
Impacts:
Poses an extreme threat to waterways, wetlands and irrigated croplands, restricts water flow in creeks, channels and drains, impedes water sports and boating access, damages pumps and irrigation equipment, increases water loss through evapotranspiration, reduces water quality, creates a favourable habitat for mosquitoes, reduces water bird and fish activity, endangers swimmers, causes the death of native, submerged water plants and fish, replaces native wetland plants
Description:
Hollow stems, dark green leaves with a distinct midrib, leaves arranged in opposite pairs along the stem, no leaf stalk , white paper-like, ball-shaped flowers appearing around mid summer, flowers carried on short (2-7 cm) stalks
Control:
Costly and difficult to control, physical removal of plant should not be attempted, herbicide control varies depending on different situations, two biological control agents established in New South Wales, see the alligator weed fact sheet for more information
Source http://www.dpi.qld.gov.au