Family

Apiaceae

Status

Water pennywort (Hydrocotyle ranunculoides) is a class 1(notifiable) noxious weed throughout NSW. It cannot be sold or propagated or knowingly distributed. It has so far been found only in West Australia, but is an invasive aquatic weed in the UK, South Africa and parts of Europe. 

Description

Water pennywort, a native of tropical Africa and the Americas, trails across the water surface or wet mud, forming dense mats. Its long runners form roots at the nodes. The emergent hairless leaves are circular to kidney-shaped, with sinus (indentation along the margin) at which the leaf stalk is inserted close to the leaf margin. Small flowers are in 5-10 flowered umbels (an umbrella shaped cluster) with green bracts below them.

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, and nutrient enrichment will greatly aid spread.
The plant 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

There are numerous native and introduced members of the genus Hydrocotyle. The most similar is the introduced Hydrocotyle bonariensis, a common weed of beaches, which may also be found growing around the edges of lagoons behind beaches, or in temporary pools after rain. It is not really aquatic though.  The main difference from the water pennywort is that the leaf stalk is inserted near the centre of the underside of the leaf, not near the leaf margin. There is a similar native pennywort, which is aquatic, Hydrocotyle verticillata, but it also has the leaf stalk inserted on the back of the leaf.  Other semi-aquatic native pennyworts have very small leaves (2cm or less across), which may be deeply divided into 3 angular lobes (Hydrocotyle tripartita).

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.