Water Lettuce

Pistia stratiotes L.

Synonyms - Pistia occidentalis

Family: - Araceae.


Pistia is from the Greek pistos meaning liquid and refers to its aquatic habitat.

Stratiotes means soldier like but is used here to associate this plant with the overseas aquatic genus of stratiotes.

Water Lettuce because it looks like a lettuce growing in the water.

Other names:

Nile cabbage (East Africa)

Tropical Duckweed.

Water lily (Philippines)


A perennial that floats freely on the water surface with rosettes of leaves like lettuces and long feather-like roots.




First leaves:

Produced underwater.


In a rosette like a lettuce. Overlapping and ridged.

Petiole - None or short on older leaves.

Blade - Pale yellow green, spade shaped to broadly egg shaped or fan shaped, 25-150 mm long by 25-80 mm wide, 6-15 veined radiating like a fan and forming ribs, thickened near the base. Tip flat or with a broad shallow notch. Hairy, becoming a dense mat towards the base. Less hairy on the under-surface. Hairs trap air, and repel water to help the plant float.

Sheath - A small pocket which gives rise to stolons.


Very short with one to several stolons, up to 600 mm long, ending in a new plant and eventually breaking down. Stolons form from small buds in pockets. Roots at the nodes.

Flower stem - None or short.

Flower head:

Lily type flower on a short, hairy, up to 15 mm long, or no stalk(peduncle). Small and inconspicuous spike, partly joined to the constricted spathe in the centre of the plant in leaf axils.

Spathe(floral leaf), 7-40 mm long, 5 mm wide, greenish white, funnel shaped, split down one side at base and spreading at the top. Hairy outside, hairless inside.

Spike or column, with a fleshy axis(spadix), shorter than spathe.


Male flowers - 2-8 per spike consisting of a single anther and appearing as a ring around the top of the spadix. Female flowers - one, at the base of the spadix. A cup shaped membrane separates male and female flowers which prevents self pollination by exposing the female before the male.

Ovary - 1 celled. Many ovules. Style short, thick remains attached to the fruit. Stigma disc like, hairy, short, thick, remains attached to fruit.

Perianth - none.

Stamens - Reduced to anthers.

Anthers - 4 celled. No stalks. Releases pollen to the outside by four pores.


Green, egg shaped to oval, 5-10 mm long, berry-like with few to many seeds. Opens by a jagged tear.


Seed doesn't form in cooler climates of WA and NSW. Elsewhere, 4-15 tiny 2 mm long seeds are produced. Initially green, then brown when ripe, oblong to egg shaped, with a flat, disk like, depressed top that contains an air chamber. Thick wrinkled seed coat.


Free floating. One to several stolons. Rhizome short, up to 50 mm with many roots. Dangling roots, unbranched, up to 1000 mm long by 2-7 mm thick, and have a dense covering of secondary roots to 40 mm long, so they look like large feathers in the water. Secondary roots have a conspicuous root cap.

In paddy situations it will take root in the soil.

Key Characters:

Aquatic, free floating perennial. Overlapping, ridged, lettuce like leaves.


Life cycle:

Perennial. Seeds float for a short time then sink to the bottom and germinate 20-40 days later if there is light and temperatures greater than 20 degrees Celsius. Seedlings produce a leaf then float to the surface. A rosette of leaves is formed and several stolons up to 600 mm long. A new daughter plant forms at the end of the stolons and produces a rosette and several more stolons. Stolons between rosettes eventually break. Flowers are only formed in dense infestations. They are wind pollinated and take about a month to mature.


Frost kills it.

Intolerant of salty water.

Suffer nitrate toxicity at levels above 5.2 g/L.


By seeds and stolons.

Flowering times:

Hasn't flowered in WA.

Seed Biology and Germination:

Seed can survive for 2 months in water at 4oC and a few weeks in ice at -5oC.

Seed has an after ripening period of 20-200 days.

They require light and temperatures above 20oC to germinate.

Vegetative Propagules:

Stolons form daughter plants at their tips and break off.



Population Dynamics and Dispersal:

Long distance dispersal is usually by the dumping of intentionally grown plants in aquaria or ponds.

Local spread is mainly by stolons breaking and releasing daughter plants, which move with the water flow.

In dense infestations in warmer areas it is also spread by seed and seedlings which floats on water.

Origin and History:

Tropical and sub-tropical areas. Probably native to the top end of the Northern Territory.

First recorded in the NT in 1887.



Caversham and around Perth in WA.


Lakes. Slow flowing streams. Warm reservoirs and dams. Rice paddies.


Tropical and sub-tropical. Frost free areas.



Plant Associations:



Ornamental. Aquarium plant.

It has a good fodder value and is palatable to pigs but not cattle. It requires at least partial drying before feeding.

It can produce 72 tons of dry matter per hectare and has potential for methane production.

It grows vigorously in polluted water and has potential for purifying water contaminated with some heavy metals. It absorbs mercury but is not tolerant of high levels.


Forms thick mats, which obstruct streams and dams.

Spreads quickly over lakes and streams interfering with their use.

The fish and water fowl environment is disrupted because it reduces light penetration, oxygen concentrations and pH of the water.

It makes an ideal breading ground for mosquitoes including those that carry malaria and filariasis.

It competes with rice.


Not recorded as toxic.


Noxious weed of NSW, NT, QLD, WA, TAS.

Management and Control:

Mechanical removal and drying is effective for small infestations but expensive for large infestations. Autumn application of diquat with wetting agent provides good control whereas regrowth often occurs after spring applications. Special formulations of diuron, glyphosate and a granular 2,4-D have also given good control.


Very low, the smallest of infestations should be dealt with promptly before it spreads.

Eradication strategies:

As above.

Herbicide resistance:

Biological Control:

A number of organisms have been used to obtain control biologically. The Neohydronomus pulchellus weevil has been used effectively in Queensland.

Related plants:


Plants of similar appearance:

Water Hyacinth (Eichhornia crassipes)


Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P19-20.

Ciba Geigy (1982) Grass Weeds 3. CIBA GEIGY Ltd, Basle, Switzerland. P86. Diagrams.

Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P20. Photo.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #984.1.

Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). P733.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P38-41. Photos.


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