Dense waterweed

Native to Brazil, dense waterweed is an aquatic plant that can form submerged masses that hinder water flow and displace native plants. Dense waterweed has caused many problems overseas, particularly in the USA. In Australia, it has invaded a number of waterways, where it is easily dispersed from broken stem pieces.

Dense waterweed is common in home aquariums and ponds. Illegal dumping of aquarium contents may have led to waterway infestations.

Dense waterweed is not a prohibited or restricted invasive plant under the Biosecurity Act 2014.

Scientific name

Egeria densa

Similar species

  • Elodea canadensis, Hydrilla verticillata

Description

  • Submersed aquatic plant growing up to 4m below water surface, with stems that can be more than 2m.
  • Roots attach firmly to silt or soil.
  • Leaves are up to 4cm long with tiny serrations on margins, grow in whorls of 4 or 5 (up to 8), mostly densely clustered at branch ends near growing tips.
  • Flowers are 2cm in diameter and have 3 white petals.

Habitat

  • Found in flowing and standing water bodies.
  • Prefers clear water.

Distribution

  • Large infestation in Lake Baroon on Sunshine Coast.
  • Found in some South East Queensland creeks.
  • Not known to be widespread in Queensland.

Life cycle

  • Reproduces mainly through stem fragments.

Affected animals

Native aquatic animals

Impacts

Environmental

  • Forms dense, submerged masses that can seriously hinder water flow.
  • Displaces native aquatic plants.

Economic

  • Interferes with irrigation equipment, hydro-electric activity and potable water supplies.

Social

  • Impedes fishing, boating and swimming.

How it is spread

  • Spread by stem fragments through water currents and humans.

Control measures

Physical control

  • Hand-pulling, cutting and digging with machines can be effective.
  • All methods need to be thorough to prevent regrowth from pieces.
  • Infestations should be controlled while small.

Herbicide control

  • No herbicides registered.

Biological control

  • No known biological control agents.

Legal requirements

  • Dense waterweed is not a prohibited or restricted invasive plant under the Biosecurity Act 2014. However, by law, everyone has a general biosecurity obligation (GBO) to take reasonable and practical steps to minimise the risks associated with invasive plants and animals under their control.
  • Local governments must have a biosecurity plan that covers invasive plants and animals in their area. This plan may include actions to be taken on certain species. Some of these actions may be required under local laws. Contact your local government for more information.

Further information