Family: Araceae

Status: 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:
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.

Preferred habitat and impacts:
Fresh water bodies such as farm dams, lagoons on river floodplains, rivers and creeks. Still or slow flowing water is preferred. Water lettuce is frost tender and unlikely to thrive on the south coast. It is a native of the Northern Territory which has been introduced to Queensland and NSW.
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. However, many aquatic species have sticky seed which can adhere to the feathers or feet of water birds, and hence be spread long distances. Many will spread from broken-off pieces or whole plants being moved on boats or fishing equipment from an infested to a clean water body.

Look-alikes:
Identifying aquatic weeds is difficult. There are many native look-alikes. Get suspicious plants identified by a specialist. Many native water plants will spread in a weedy way if the nutrient level in the water body is increased or the temperature raised. This may not be undesirable, since these plants will use up nutrients which might otherwise feed a toxic blue-green algae bloom.
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.