Family: Pontederiaceae

Status: Water hyacinth is listed as a W1 (notifiable) noxious weed in Eurobodalla and Bega Valley Local Government Areas, as W2 (must be continuously suppressed and destroyed, but not notifiable) in the Shoalhaven and is not listed in the Illawarra.

Description:
Water hyacinth is unattached and floats on the water surface, forming dense mats. It is quite a substantial plant, varying from 10cm to 1m in height depending on nutrient levels. Leaves are glossy, fleshy and spoon-shaped. Flowers are in spikes, large and mauve.

Preferred habitat and impacts:
Fresh water bodies such as farm dams, lagoons on river floodplains, rivers and creeks. Still or slow flowing water is usually preferred, but it can spread downstream on flowing water.
Hyacinth 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, reduces water quality and blocks pumps.

Dispersal:
Dumping of aquarium or ornamental pond plants is often the means of spread for aquatic weeds. 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, or can be washed out of lagoons into river systems during floods.

Look-alikes:
The broad fleshy leaves, floating habit and showy mauve flowers of water hyacinth are unmistakable. A related weed is blue pickerel weed (Pontederia cordata), which is often planted as an ornamental around garden water features. It is a taller plant which is rooted in soil at the water’s edge. It also has glossy spoon-shaped leaves and spikes of much smaller mauve flowers.

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 hyacinth 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 required, 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. Notify your local weed control authority (usually Council) and take their advice on control methods.