Family: Salviniaceae

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:
Salvinia is an aquatic fern which floats on the water surface. Initially leaves are round, 1-4 cm long, with the upper surface covered in papillae (like a cat’s tongue) and the lower surface with a dense mat of brown hairs. Root-like structures dangle below the leaves. As the infestation becomes more crowded the leaves fold upwards into a shape rather like a sheep or goat’s cloven hoof.

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.
Salvinia blankets 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, reduce water quality and block pumps.

Dispersal:
Dumping of aquarium or ornamental pond plants is often the means of spread for aquatic weeds. Salvinia molesta is a sterile hybrid and only reproduces vegetatively, from broken-off pieces or whole plants being moved on boats or fishing equipment or washed from one water body to another in floods.

Look-alikes:
Salvinia is variable in appearance, with young leaves lying flat on the water, becoming folded as plants become more crowded. There are no very similar natives, although there are a few small floating natives such as the red-coloured fern azolla (Azolla pinnata and A. filiculoides) which can cover large expanses of water from time to time. It has finely divided leaves. Water lettuce (Pistia stratiotes) is the most similar plant, but has a rosette of erect lettuce-like leaves rather like a mignonette lettuce. It is native to the Northern Territory, but introduced in Queensland and NSW.

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. Salvinia can be raked to shore or pulled in with an encircling rope, and piled on the shore above flood reach under plastic, where it 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.