Family: Amaranthaceae

Status: A Weed of National Significance (one of Australia’s top twenty weeds). Listed as noxious in all four South Coast Local Government Areas in category W1, meaning the local weed authority (usually Council) must be notified immediately if an infestation is discovered.

Description:
A large perennial herb which is rooted at the water’s edge and spreads out as a floating mat over the water surface, and over the adjacent moist ground as well. Mats may be up to 1m thick. Stems are spreading, hollow and may be green, yellow or red. Leaves are in opposite pairs, narrow (2-7cm long x 4-40mm wide), slightly fleshy and with a waxy surface. Flowers are tiny, white, papery and in spherical heads about 1cm in diameter, on 2-6cm stalks in the leaf axils.

Preferred habitat and impacts:
Fresh water bodies, preferably still. Also spreads over adjacent moist ground to some distance from the water’s edge. Can tolerate a degree of salinity, and total immersion for periods of a few days.
Alligator weed can blanket the water surface reducing light levels, temperature and oxygen in the water below. This has profound effects on communities of native plants and animals in the water. It also interferes with animal access for drinking water, human access for swimming and boating, reduces water quality and blocks pumps.

Dispersal:
Does not produce seed in Australia, but grows readily from broken off sections of stem, which may be moved around on machinery or boats, or by floods, wind or waves. Slashing of infestations which are growing on land will only spread the infestation for this reason.
Alligator weed has sometimes been deliberately spread in mistake for a similar but non-invasive introduced water plant Alternanthera sessilis, which is used as a culinary herb in Asia.

Look-alikes:
There are no similar natives of the size of alligator weed but a very much smaller native, lesser joyweed (Alternanthera denticulata) which has similar leaves and flowers, grows on mud around the edges of dams and creeks.
The similar exotic Alternanthera sessilis has solid, not hollow stems, and flower clusters which are right in the leaf axils, not on short stalks.

Control
Because of the ease of breaking plants up and spreading them during mechanical removal, herbicide is the preferred method of control, but a permit will be required from the Environmental Protection Agency to apply any herbicide to a water body. Only a limited number of herbicides are registered for use over water. If you suspect you have an outbreak of an aquatic weed, notify your local weed control authority (usually Council) and take their advice on control methods.