Water lettuce

Water lettuce is a free-floating plant aquatic weed found in tropical countries worldwide, including Asia, Africa and equatorial America. Its country of origin is not clear. Water lettuce was introduced to Australia as an aquarium and water-garden plant.

Water lettuce rapidly forms dense infestations that cover the surface of entire rivers, dams and irrigation channels. It affects water flow, damages native ecosystems, and impedes recreational use of water bodies.

Water lettuce is a restricted invasive plant under the Biosecurity Act 2014.

Scientific name

Pistia stratiotes

Other names

Nile cabbage, water cabbage, tropical duckweed

Description

  • Free-floating, spongy, aquatic perennial, resembling small, floating, open head of lettuce.
  • Leaves are fan-shaped, covered with hairs.
  • Roots are tufted, unbranched, feathery, up to 80cm long.
  • Flowers are small, green, 10-20mm long.
  • Fruit is egg-shaped, greenish, 1cm across, containing 4-15 seeds.
  • Seeds are oblong, 2mm long, tapered at each end.

Habitat

  • Prefers stationary or slow-flowing water.
  • Sensitive to frost.
  • Grows best in water with high nutrient concentrations.

Distribution in Queensland

  • Found in areas throughout eastern Queensland.
  • Widespread in Warrego River at Cunnamulla.

Life cycle

  • Seeds germinate in muddy bottoms in late November-early December and float to surface as seedlings.
  • Flowering and reproduction begins early in plant's life, about fourth or fifth leaf stage, when plant densities are high.
  • Propagation is by seed or by 20cm long stolons (runners) that produce daughter plants; propagation by stolons is most prevalent form of reproduction
  • Flowering occurs mostly during summer and early autumn.

Affected animals

Native aquatic animals; Humans

Impacts

Environmental

  • Restricts water flow and increases water loss.
  • Large infestations damage wildlife habitats.
  • Serves as breeding ground for mosquitoes.
  • Transforms aquatic ecosystems.
  • Shades out native aquatic plants.
  • Reduces oxygenation of water.

Economic

  • Interferes with irrigation and stock watering.

Social

  • Large infestations interfere with boating.
  • Degrades quality of swimming and fishing.

How it is spread

  • Spread by daughter plants and seeds.
  • Spread mostly by water movement.

Control


Physical control

  • Remove by hand for small areas or when numbers are low, before flowering and seeding.

Mechanical control

  • Waterweed harvesters may be used for larger areas.

Herbicide control

  • Herbicides are effective.
  • Spraying results in quicker death in warmer months.

See the Water lettuce fact sheet (PDF, 1.4MB) for herbicide control and application rates.

Biological control

  • Weevil Neohydromonus affinis has been introduced and exerts good control in some areas.
  • Weevils have proved effective in dams from Bundaberg to Brisbane. Weevil life cycle takes about 3 months. Eggs are laid in fleshy leaves and larvae tunnel through plant tissue. Openings aid entry of fungi and bacteria, causing tissue to rot.
  • Biological control is most effective on large infestations, but may take many years to achieve satisfactory control. To establish an effective breeding population of biological control agents, place infested plants in an area where water lettuce is concentrated.
  • Your local government office or local Biosecurity Queensland officer can provide protocols and more information.

Legal requirements

  • Water lettuce is a restricted invasive plant under the Biosecurity Act 2014.
  • It must not be given away, sold, or released into the environment without a permit.
  • The Act requires everyone to take all reasonable and practical steps to minimise the risks associated with invasive plants and animals under their control. This is called a general biosecurity obligation (GBO).
  • At a local level, each local government must have a biosecurity plan that covers invasive plants and animals in its 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.

More information