Quick Facts

Quick facts

• Water Lettuce (Pistia stratiotes) is a stoloniferous, free-floating, freshwater perennial herb, considered native in tropical Australia but introduced farther south.
• It has overlapping, spongy, ribbed, velvety leaves forming a lettuce-like rosette.
• It can reproduce prolifically, mainly by forming daughter plants on the end of stolons.
• Under favourable warm to tropical conditions, Water Lettuce can rapidly expand to form obstructive mats, which interfere with water flow, irrigation and water storage systems, navigation and recreation activities, and can also have habitat and environmental impacts.

What Does It Look Like?

What is it?

Water Lettuce (Pistia stratiotes) is a lettuce-like stoloniferous, free-floating, mat-forming, perennial aquatic herb with leaves in rosettes up to 30 cm in diameter. It may also be attached to mud. The horizontal runners (stolons) are up to 60 cm long and produce new plants at their tips, while at the base of the rosettes there are trailing feathery roots to 1 m long that hang below the water surface. The overlapping leaves are spoon-shaped to obovate or wedge- to fan-shaped (being broad at the top and narrowed towards the base), pale green, yellowish green or grey-green, 2–17 cm long, 1.5–8.5 cm wide, spongy, longitudinally ribbed, softly velvety hairy, and with the leaf stalk much shortened and inflated.

An inconspicuous, 7–20 mm long, whitish green flower head grows amongst the leaf bases. Within a whitish tubular or funnel-shaped bract (spathe), that opens out towards the top and is slit down one side, there is a central column or spadix which carries a pistil (female part) below a whorl of stamens (male flowers). A cup-shaped membrane separates the male and female flowers.

The fruit is a greenish, ovoid to ellipsoid berry 5-10 mm long. The mature seeds are pale reddish brown, about 2 mm long, with a thick wrinkled seed-coat. They are largely enclosed in a cylindrical, buoyant, spongy material with an air chamber (Aston 1973; Sainty & Jacobs 1981, 2003; Stanley & Ross 1989; Hay 1993; Cowie et al. 2000; Parsons & Cuthbertson 2001; Sainty & Associates 2003).

For further information and assistance with identification of Water Lettuce contact the herbarium in your state or territory.

Flower colour

• White
• Green

Growth form (weed type/habit)

Herb, Aquatic

Where it currently grows? Preferred habitat

Water Lettuce grows best in still and slow-flowing water, in freshwater lakes, ponds, streams, wetlands, farm dams and reservoirs. In northern Australia, Water Lettuce is naturally found on wetter back-water swamps and associated billabongs, often in shallow water or on mud (Cowie et al. 2000). It has become a weed mostly in eastern Australia where it has invaded and caused problems in irrigation canals, constructed water-storage dams and natural water-bodies. Water Lettuce can survive long periods on wet muddy banks of water-bodies or in damp locations such as roadside culverts (Osmond & Johnson 2006).

Under tropical conditions in nutrient-rich water, Water Lettuce will produce luxuriant growth and expand rapidly to form obstructive mats (Sainty & Associates 2003). It tolerates temperatures between 15 °C and 35 °C, but optimum temperatures for growth are 22–30 °C. Water Lettuce is susceptible to frost, which limits its distribution in temperate areas (Osmond & Johnson 2006).

Are there similar species?

Water Lettuce (Pistia stratiotes) is similar to the introduced aquatic weeds Water Hyacinth (Eichhornia crassipes) and Salvinia (Salvinia molesta) in forming floating obstructive mats, but it is distinguished by its characteristic ribbed, spongy, velvety leaves forming lettuce-like rosettes and long fleshy petioles (leaf stalks) (Navie 2004, Kodela 2007, pers. comm.).

Why Is It A Weed?

What are its impacts?

Although considered native to northern Australia, Water Lettuce is regarded as a weed in many parts of Australia where it can have impacts on the environment, agriculture, recreation and water supplies.

Water Lettuce is regarded as one of the world's worst weeds (Holm et al. 1977). In Australia it is potentially a serious weed of warm-watered, still to slow-flowing waterbodies. Once established, it spreads quickly over the entire surface of freshwater lakes, ponds, sluggish streams and rivers, canals and other bodies of water (Parsons & Cuthbertson 2001).

Problems occur when the stolon-connected rosettes form extensive dense mats, which obstruct waterways and dams, impeding water traffic, hydroelectric systems and irrigation, and interfering with recreational activities such as boating, swimming and fishing. They hinder flood control efforts by blocking and disrupting water flow in drainage channels. Children and livestock may drown if they become entangled in the roots of a heavy infestation (Land Protection 2006).

Agriculture: There can be an increased water loss in summer in reservoirs by transpiration. Infestation by Water Lettuce can also pollute water with rotting plant material, affecting people, fish, livestock and wildlife (Land Protection 2006).

Native ecosystems: Invasion of habitats by Water Lettuce disrupts habitats and ecosystems. The dense plant mass reduces light penetration, oxygen concentrations and pH levels in the water, thus destroying the habitat of fish and waterfowl and can lead to a loss of biodiversity as it out-competes and displaces native water plants. It can also be a serious competitor in rice paddies, taking root in the soil and competing for nutrients and spaces (Parsons & Cuthbertson 2001).

Water Lettuce may form a shelter for, and a breeding site of, disease-carrying mosquitoes, especially a hazard where mosquitoes carry parasites responsible for malaria, Ross River Fever and other diseases (Sainty & Jacobs 1981; Parsons & Cuthbertson 2001; Land Protection 2006).

Within its natural range on the coastal floodplains of the Northern Territory, Water Lettuce plays an ecological role in initiating the formation of floating grass mats important for wildlife habitats; including nesting places for the saltwater crocodile (Gillett et al. 1989; Hills 1998). In the Northern Territory it is kept in check partly by the moth Nymphula tenebralis which lays its eggs on the leaves and the larvae then feed on the plant (Gillett et al. 1989; Hills 1998). Water Lettuce should only be treated as a noxious weed in made-made water storages in the Northern Territory (Gillet et al. 1989).

How does it spread?

Water Lettuce reproduces vegetatively and by seed.

The seeds float on the water surface and can be dispersed some distance by water flow before they sink and germinate. The buoyant seedlings can also be moved by water and assisted by wind to spread the infestation over a wide area. (Parsons & Cuthbertson 2001; Weber 2003).

The primary colony increases its size and density by developing daughter plants at the end of stolons. Further spread occurs when stolons break, freeing daughter plants from the parent mass and enabling them to disperse and form new colonies (Parsons & Cuthbertson 2001). Broken-off floating rosettes will be moved in water by stream currents, wind and floods. Broken pieces, buoyant seeds and seedlings or whole plants can also be spread by boats or fishing equipment from an infested area to a clean water body (Osmond & Johnson 2006).

Propagules of Water Lettuce may also be dispersed by migratory waterfowl and waders, which is a possible explanation for how it may have arrived in the Northern Territory (Gillett et al. 1989).

Water Lettuce has also spread through the aquarium plant trade (Osmond & Johnson 2006) and presumably subsequent dumping from aquariums or fish ponds into waterways, like many other invasive water plants.

What is its history in Australia?

In Australia, Water Lettuce was first recorded in the Northern Territory in 1887 (Gillett et al. 1989; Parsons & Cuthbertson 2001). It is considered native to the Northern Territory and possibly northern Queensland, but Water Lettuce has been introduced to some areas as an ornamental aquarium and pond plant, and has become a serious weed in lakes, dams, ponds and slow-moving water in tropical to warm temperate regions (Richardson et al. 2006). Since about 1967 it has been reported in Queensland as a weed of open water (Kleinschmidt & Johnson 1977). It is thought to have been introduced to New South Wales rivers and dams via eel traps from Queensland (Osmond & Johnson 2006).

How To Manage It?

Best practice management

Water Lettuce should not be spread to uninfested waterways and water storages bodies. Plants should not be sold from nurseries or garden centres (banned in most states and territories), or exchanged between aquarium owners.

Chemical control: is usually less expensive and more practical than physical removal. Herbicides used to control Water Lettuce that are appropriate for aquatic environments are described by the Department of Agriculture and Food, Government of Western Australia (no date), Rivers III (undated) and Parsons & Cuthbertson (2001).

See the Australian Pesticides and Veterinary Medicines Authority (APVMA) for chemical information http://www.apvma.gov.au.

Non-chemical control: Physical control: Physical removal is effective, but only practical for small infestations, emphasising the advantage of early detection for controlling Water Lettuce (see Parsons & Cuthbertson 2001; Rivers 2002; ISSG 2005; Osmond & Johnson 2006).

Biocontrol: Water Lettuce has also been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. Biological control through insects (including weevils and nocturnal moths) and fungal pathogens has been effective. Neohydronomus affinis (sometimes reported as another species N. pulchellus) is a leaf weevil from South America imported into Australia as a potential biological control agent and released into the field in 1982. It controls Water Lettuce effectively in tropical regions (to latitude 22° S), but in cooler regions its effectiveness fluctuates with seasonal conditions. (Harley et al. 1990; Rivers III 2002). In the Northern Territory it is kept in check partly by the moth Nymphula tenebralis which lays its eggs on the leaves and the larvae then feed on the plant (Gillett et al. 1989; Hills 1998). For further details on biological control see DeLoach (1976), Harley et al. (1984, 1990), Gillett et al. (1989), Thompson & Habeck (1989), Waterhouse (1994), Parsons & Cuthbertson (2001) and Dray & Center (2002).

For further information on general management and control strategies of Water Lettuce see Gillett et al. (1989), Land Protection (2006) and Osmond & Johnson (2006).

Does it have a biological control agent?

YES

When does it grow? (lifecycle/growth calendar)

Water Lettuce flowers are probably produced throughout the year (Hussey et al. 1997). Seeds mature about 30 days after fertilisation, and germinate in light at temperatures above 20 ^oC about 20-40 days after sinking to the bed of the waterbody (Parsons & Cuthbertson 2001). Germination has been reported to occur in late November to early December (Land Protection 2006). Seedlings, aided by a buoyancy chamber in the seed, float to the surface after the primary leaf is exposed. The young plants produce several stolons which terminate in a new rosette or daughter plant which in turn forms stolons and further daughter plants. Flowers form on mature plants and are wind-pollinated (Parsons & Cuthbertson 2001).

In the Northern Territory, populations of Water Lettuce appear to undergo periodic growth explosions, mostly during the late dry season (Cowie et al. 2000). In Florida in the United States, Water Lettuce exhibits seasonal growth with high rosette densities during winter and spring and low densities during late summer and early autumn, while leaf size, leaf density per rosette and total biomass increase during spring and summer then begin to decline during late autumn (Dray & Center 2002).

Where Is It Found?

Which states and territories is it found?

NSW, NT, QLD, WA

What areas within states and territories is it found?

There is evidence that Water Lettuce is native to the Northern Territory and possibly northern Queensland, but introduced in other parts of eastern Australia (Gillett et al. 1989; Cowie et al. 2000). It is widespread but relatively uncommon in the northern Top End, where it is known from scattered localities on the major floodplains from the Reynolds River to the Arafura Swamp (Cowie et al. 2000). Outside the Northern Territory it occurs as scattered colonies along the east coast north of Sydney, as well as the inland pastoral districts of Darling Downs and Leichhardt in Queensland (Parsons & Cuthbertson 2001). Small colonies have been reported around Perth in Western Australia (Hussey et al. 1997; Parsons & Cuthbertson 2001).

Where does it originate?

Water Lettuce is native to tropical and subtropical regions of the world, including northern Australia. Its origin is not clear, but Water Lettuce is now widely distributed, occurring in Asia, Africa, America, the Caribbean, the Netherlands, several Indian Ocean and Pacific Islands, Australia and New Zealand (Parsons & Cuthbertson 2001; Weber 2003). It is a serious weed in Africa, southern Asia, south-eastern United States and islands of the Caribbean (Sainty & Jacobs 1981; Hills 1998; Dray & Center 2002).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?

NO

Where is it a declared weed?

ACT, NSW, NT, QLD, WA

Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?

NO

Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?

NO

Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Pistia stratiotes

Other scientific names (synonyms)?

Does it have other known common name(s)?

Nile Cabbage, Water Lily