Water lettuce (Pistia stratiotes, L. 1753)

Identity

Water lettuce (Pistia stratiotes L., 1753), also known as Water cabbage, Shellflower or Nile cabbage, is a free-floating aquatic plant and the sole species of Pistia in the Araceae family. ¹ Its velvety leaf rosette floats without rooting in a substrate, unlike most terrestrial or pot-grown aroids. Carl Linnaeus described it in 1753 from populations observed on the Nile near Lake Victoria, the geographical association behind the name Nile cabbage. ⁹

The epithet stratiotes derives from the ancient Greek stratiôtês, meaning soldier. It refers to the sword-shaped leaves of Stratiotes, an unrelated aquatic genus with which Linnaeus compared the plant. ⁹ Water lettuce describes the resemblance of its floating rosette to a head of lettuce; it has no botanical relationship to the vegetable.

Recognition

A floating rosette holds pale green, velvety, obovate leaves 5–20 cm (2–8 in) long, with 5–13 prominent parallel veins. ⁹ Fine hairs trap air bubbles, helping the leaves remain at the surface. ²

Feathery roots hang freely below the rosette and may reach 50 cm (20 in) in deep water. They provide relative anchorage and absorb dissolved nutrients. ⁹ Individual rosettes usually stand 8–15 cm (3–6 in) high and span 15–45 cm (6–18 in), although a continuous mat can cover several square meters within weeks. ²

Flowers remain almost hidden in the rosette center. A tiny white, green and yellow spadix, enclosed by a reduced spathe, carries a single female flower surrounded by male flowers. ²

Useful taxonomy

The monotypic genus Pistia contains one species, placed in the isolated tribe Pistieae within the Aroideae subfamily. ¹ Its unusual position helps explain the many regional names once assigned to it and now treated as synonyms:

• Pistia africana C.Presl, an African name;
• Pistia amazonica C.Presl, an Amazonian name;
• Pistia brasiliensis Klotzsch, a Brazilian name;
• Pistia natalensis Klotzsch, a regional African name;
• Pistia obcordata Schleid., a historical synonym;
• Pistia spathulata Michx., another historical synonym;
• Pistia texensis Klotzsch, a Texan name.

These seven names, published from Africa, South America and then the United States between 1803 and 1878, initially suggested several continent-specific species. Botanical revisions ultimately united them under one cosmopolitan species, Pistia stratiotes L. ¹

Habitat and range

Whether Water lettuce originated in Africa or South America remains disputed. It was first described from the Nile, yet the long co-adaptation of several South American plant-feeding insects also supports a South American origin. ³ This uncertainty reflects its long history of dispersal: today it is pantropical, occurring on every continent except Antarctica. ⁹

Aquarium and ornamental pond sales remain the principal introduction route beyond its native range. Discarding aquarium water or unwanted plants into a natural pond can establish a population. ³ The species has been recorded in 44 US states. ³

Metropolitan France treats this nonnative plant as a regulated invasive alien species, found mainly in the south and around the Mediterranean. Records also include Guadeloupe, Martinique, Mayotte, Réunion, French Guiana and French Polynesia. ⁵ Since August 2, 2024, European Union restrictions associated with the list of invasive alien species of Union concern prohibit its importation, cultivation and sale throughout the EU. ⁶

Life cycle and reproduction

Stolons drive colonization far more than seeds. Horizontal runners grow from the leaf bases and produce daughter rosettes at their tips, allowing biomass to double in 10–15 days in warm, nutrient-rich water. ^{11 10}

Such rapid doubling lets a mat cover an entire pond within a few weeks. Light and dissolved oxygen then decline in the water beneath it. ³

Warm water at 22–30 °C (72–86 °F) favors optimum growth; below 10 °C (50 °F) growth slows markedly, and no frost is tolerated. ⁵ In temperate climates it consequently behaves as a pond annual, dying in fall and requiring replacement the following spring wherever cultivation remains permitted. ²

Uses and risks

Floating rosettes are grown mainly as ornamentals in ponds and aquariums, shading the surface and limiting algae. ² Water lettuce is also studied as a hyperaccumulator of heavy metals such as cadmium, copper and mercury, and used in phytoremediation of some wastewater and pesticide-contaminated water. ^{9 7}

Indian traditional medicine uses dried leaves or poultices for skin disorders and fungal infections. Extracts show antifungal, diuretic and antioxidant effects in laboratory studies, but these uses lack clinical validation. ⁷

Chewing the leaves can cause mouth irritation, burning and nausea because they contain calcium oxalates, particularly concerning in children, even though the plant is not classified as toxic by ingestion. ² Its ecological risks are better documented: dense mats lower dissolved oxygen, obstruct navigation and provide egg-laying habitat for mosquitoes that transmit disease. ⁴

Varieties, look-alikes and related plants

Aquatic nurseries mainly supply the wild form, alongside 'Ruffled', a compact selection with more crinkled leaves suited to small ponds. This is a horticultural selection of Pistia stratiotes, without a separate botanical rank.

Other floating aquatics can cause confusion:

• Water hyacinth (Eichhornia crassipes) grows taller, with showy purple flowers and swollen leaf stalks that act as floats;
• Duckweed (Lemna minor) is tiny and lacks an individual rosette visible to the unaided eye;
• Salvinia has rounded leaves with brushlike hairs rather than an elongated rosette.

Look for an obovate rosette with parallel veins, no conspicuous flower or inflated leaf stalk, and feathery submerged roots. This combination separates Water lettuce from water hyacinth and duckweed. ²

Main sources

• Plants of the World Online / Kew, Pistia stratiotes L.
• NC State Extension Gardener Plant Toolbox, Pistia stratiotes
• USGS Nonindigenous Aquatic Species, Pistia stratiotes species profile
• IUCN Global Invasive Species Database, Pistia stratiotes
• Office français de la biodiversité, Water lettuce (Pistia stratiotes)
• EPPO Global Database, Pistia stratiotes (PIIST) reporting
• PharmaTutor, Pharmacological updates of Pistia stratiotes
• IUCN Red List assessment, Pistia stratiotes
• Wikipedia FR, Pistia stratiotes
• Mississippi State University Extension, Water Lettuce (Pistia stratiotes)
• Revista de Biología Tropical, density-dependent allocation in Pistia stratiotes