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Water Lettuce

water lettuce1

water lettuce2

water lettuce3

 


family

Araceae

status

Water lettuce (Pistia stratiotes) is listed as noxious in class 1 throughout NSW, meaning the Local Control Authority (usually Council) must be notified immediately if an infestation is discovered, the plant must be eradicated and the site kept free of the weed. It cannot be sold or propagated or knowingly distributed.

description

Water lettuce floats on the water surface and will also grow on mud around the water’s edge. It spreads by stolons (white root-like structures which link plants together at their bases). Each plant forms a rosette with the leaves held erect, rather like a mignonette lettuce. Leaves are pale yellow-green and fan-shaped, with 6 or more prominent veins on the underside. They are densely covered with short white hairs which trap air and provide buoyancy. Flowers are inconspicuous and are followed by small berry-like fruits, 5-8mm across.  Photos of this weed are provided by RG & FJ Richardson of www.weedinfo.com.

preferred habitat and impacts

Still or slow flowing fresh water is preferred, such as farm dams, lagoons on river floodplains, rivers and creeks. Water lettuce is frost tender and unlikely to thrive on the south coast. It may be native to the Northern Territory (having a pan-tropical distribution including Asia, Africa and equatorial America) but has been introduced to Queensland and NSW, where there are fewer natural controls on its spread.
Water lettuce could 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 may interfere with animal access for drinking water, human access for swimming and boating, reduce water quality and block pumps.

dispersal

Dumping of aquarium or ornamental pond plants is often the means of spread for aquatic weeds. Water lettuce reproduces both vegetatively, by development of daughter plants on the stolons, and from seed. Broken off plants may float long distances and establish downstream, and seed is spread in water.

look-alikes

There are no very similar plants to water lettuce.

control

Most importantly, do not dump unwanted aquatic plants into water bodies, or grow species with weed potential in ornamental ponds or aquaria. Some invasive water plants are still sometimes sold by nurseries or pet shops. If you notice this, report the instance to Council, so that the proprietors can be advised that it is illegal to sell these plants.
Once an infestation is established, and has been definitely identified, there are two options, mechanical or chemical control. Floating plants such as water lettuce can be raked to shore or pulled in with an encircling rope, and piled on the shore above flood reach under plastic, where they will break down rapidly. For large infestations herbicide may be necessary, 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.