Plant 143

Pistia stratiotes L. (Araceae)


Water lettuce

Pistia stratiotes is a monocotyledonous aquatic plant originally from East Africa, with populations concentrated around Lake Victoria and the River Nile. It is an aggressive species capable of rapid reproduction and is considered an invasive weed; it has spread to many tropical and sub-tropical regions across the planet.

Pistia reproduces very effectively by sending out stolons, a type of stem, across the surface of the water. Plantlets quickly develop at the end of these stolons and remain attached to the parent plant until roots have formed. In cool climates it behaves like an annual, reproducing from seed each year.

It can form huge floating mats of vegetation, more than 10 metres wide and over half a metre thick. These can support other plant life and provide an environment for birdlife and fish fry. There are even reports of large, stranded mammals seeking refuge on these rafts of water lettuce. Pistia stratiotes colonises slow-moving waterways and is effective at purifying stagnant water.

Unfortunately, the negative impact of introduced Pistia populations on the function of native ecosystems far outweighs any benefits. The plant rapidly clogs waterways, affecting oxygen levels and preventing light from reaching the water below, in a similar manner to the fern salvinia. It outcompetes native submerged plants and reduces the diversity and number of fish species present. This can impact heavily on human activities such as fishing and crop irrigation. Effective control of Pistia is difficult but the most common methods used are physical removal and herbicides.

Pistia is a monospecific genus in the arum family, which is most obvious when the plant's are flowering. Spathes, up to one centimetre long, cluster between the leaf bases at the centre of the plant, just above the water surface. The spathe surrounds a single female flower and a ring of male flowers. Pollination studies have been inconclusive but it is most likely to be carried out by beetles.

Pistia has evolved several buoyancy adaptations for its watery environment. Resembling a loose-leafed lettuce, plants of Pistia stratiotes drift freely across the water surface. Finely-divided roots hang in the water below floating rosettes of corrugated, spongy leaves, which contain air-filled cavities to keep the plant afloat. The velvety leaves are also covered with a dense layer of minute, water-repellent hairs. Air trapped between the hairs on the leaf surface make the plant difficult to sink and give the vibrant-green foliage a silvery appearance.

Further reading

Bown D 2000. Aroids: plants of the arum family. Timber Press.

Mayo SJ, Bogner J, Boyce PC 1997. The genera of Araceae. RBG Kew.

Kate Pritchard