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Dense waterweed

Egeria densa flower1

Egeria densa flower2

Elodea canadensis

Elodea canadensis closer

Ottelia ovalifolia





Dense waterweed or leafy elodea (Egeria densa) is listed as noxious in Class 4 throughout NSW. It cannot be sold or propagated or knowingly distributed.


Dense waterweed grows attached to the bottom with the long stems floating in the water column and leaves mostly fully submerged below the water surface, in  whorls of 3-8 oblong, undivided leaves. Flowers are white, 3-petalled and 1-2cm across. They are held above the water surface on slender thread-like stems.

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. Infestations can become dominant in still waters, crowding out native water plants. It can interfere with human access for swimming and boating, reduce water quality and block pumps.


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.


Identifying aquatic weeds is difficult. There are many native look-alikes. Get suspicious plants identified by a specialist. Many native water plants will increase 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.

Canadian pondweed (Elodea canadensis) has similar leaves but they are in whorls of only 3 and it lacks the white flowers held above the water. It is still legal to sell this introduced water plant, but it would be preferable to use local natives for ornamental ponds. The weed Lagarosiphon major is also similar (see separate entry for this weed). The native swamp lily (Ottelia ovalifolia) has similar white flowers which sit at the water surface, but its leaves, which also sit on the surface, are quite large (to about 15cm long), oval and waterlily-like.

The native pondweeds (Potamogeton species) also have long trailing submerged stems, but their leaves are simple and arranged in opposite pairs or alternately along the stems. Their small spikes of inconspicuous flowers are held just above the water surface in summer.

Native species of water milfoil (Myriophyllum species) are very similar in habit, and their leaves are either simple or finely lobed  and usually arranged in whorls of 3-6 around the stems. Their flowers are tiny, red and clustered in the leaf axils. There is also an introduced water milfoil, parrot’s feather (Myriophyllum aquaticum).  Check the cabomba entry for images of these plants.


Most importantly, do not dump unwanted aquatic plants into water bodies, or grow species with weed potential in 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. Chemical control is difficult for a plant which is growing almost entirely below the water surface. 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.