water lettuce

Pistia stratiotes
Aquatic
Basal
Simple
Green
Green
Discoloured

A free-floating plant forming lettuce-like rosettes up to 30 cm across. Its feathery roots dangle below the water surface. The leafy rosettes produce runners up to 60 cm long, which in turn produce new rosettes. Its fan-shaped leaves have several prominently ribbed veins on their undersides and are covered in white velvety hairs. Separate male and female flowers are arranged in a tiny column-like flower cluster.

Common names 
Also known as: water lettuce, Pistia, floating aroid, Nile cabbage, tropical duckweed, shell flower, water cabbage,
Family 
Araceae
Deciduous 
No
Flowering time 
Summer - Autumn
Native/Exotic 
Exotic
Origin 
This species is thought to have originated in South America (i.e. in Brazil) but is now widespread throughout the tropical and sub-tropical regions of the world (i.e. it is pan-tropical). It was probably introduced into Australia from Asia.
Notifiable 
No
State declaration 
Class 2
Council declaration 
Class C – Containment and reduction
Known distribution 

Water lettuce (Pistia stratiotes) is mainly naturalised in the coastal districts of northern and eastern Australia. It is most common in south-eastern Queensland, north-eastern New South Wales and the coastal districts of the Northern Territory. Also present in the coastal districts of northern and central Queensland, central New South Wales, and Western Australia.

Habitat 

A weed of lakes, dams, ponds, irrigation channels and slow-moving waterways in tropical, sub-tropical and warmer temperate regions.

Habit 

A freshwater (i.e. aquatic) plant consisting of free-floating rosettes of leaves (up to 30 cm across) with feathery roots (up to 80 cm long). Plants develops runners (i.e. stolons) and often form dense mats of vegetation on the water surface.

Impact and control methods 

Water lettuce (Pistia stratiotes) is regarded as a significant environmental weed in Queensland, New South Wales, the ACT and Western Australia. It was recently listed as a priority environmental weed in at least one Natural Resource Management region, and is also listed in the Global Invasive Species Database (GISD).

Stem and leaves 

The floating rosettes produce runners (i.e. stolons) from the leaf bases (i.e. axils). These runners (up to 60 cm long) are green, rounded (i.e. terete), and quite hairy (i.e. pubescent). They give rise to new rosettes and then readily detach from the parent plant. The spongy leaves are pale yellowish-green or greyish-green in colour. These leaves (2.5-15 cm long and 2-8 cm wide) are fan-shaped or slightly spoon-shaped (i.e. spathulate) and have scalloped (i.e. crenated) margins. They have 6-15 prominent ribs (i.e. veins) on their undersides and are borne on very short leaf stalks (i.e. petioles). These leaves are arranged in an overlapping fashion on a very short stem (i.e. they are densely alternately arranged into a rosette) and are covered in soft, white, velvety hairs (i.e. they are velvety pubescent). The hairs are water repellent and trap air, thereby providing buoyancy to the plant.

Flowers and fruits 

The inconspicuous flowers lack petals and arranged on a short column-like flower clusters (10-20 mm long) arising in the leaf forks (i.e. axils). These flower clusters (i.e. spadices) have separate male and female flowers (i.e. the flowers are unisexual). A single female (i.e. pistillate) flower, consisting of an ovary and style, is found at the bottom and several (2-8) male (i.e. staminate) flowers, consisting of two stamens, are located at the top. At the base of the flower cluster is a whitish coloured bract (i.e. spathe) that is 7-12 mm long and about 5 mm wide. There is also a cup-shaped membrane on the column, which separates the male and female flowers from each other. Flowering occurs mostly during summer and early autumn. The egg-shaped (i.e. ovoid) or oval (i.e. ellipsoid) fruit are greenish coloured berries (5-10 mm long) that contain 4-15 seeds. The mature seeds are light brown (about 2 mm long and 1 mm wide), oblong in shape, have a wrinkled (i.e. rugose) surface, and contain a small air chamber.

Reproduction and dispersal 

This plant reproduces vegetatively, by developing new plants along its runners (i.e. stolons), and also by seed.The seeds float and are therefore most commonly dispersed by water. Entire rosettes may also be spread by water and wind movement and by boats. They are also commonly dispersed in dumped aquarium or garden waste.

Similar species 

Water lettuce (Pistia stratiotes) is a very distinctive floating plant and is rarely confused with other species. Water hyacinth (Eichhornia crassipes) is another free-floating species that is slightly similar to water lettuce (Pistia stratiotes) when not in flower. It can be distinguished by its fleshy leaf stalks (i.e. inflated petioles) and by the absence of hairs on its leaves (i.e. its leaves are glabrous).