Water Lettuce
Pistia stratiotes
NEMBA: Category 1b
English: Water Lettuce, Water Cabbage, Nile Cabbage, Shellflower
Afrikaans: Waterslaai
Historical scientific names: Limnonesis commutata, Pistia aegyptiaca, Pistia africana, Pistia amazonica, Pistia spathulata, Pistia texensis
Pistia stratiotes, commonly known as Water Lettuce, is a free-floating aquatic plant that resembles an open head of lettuce or cabbage. It is easily identified by its rosette of thick, velvety, ribbed, light green leaves and its dense, fibrous root system that hangs beneath the water surface.
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Origin:
Thought to be native to the Nile River in Africa, but its exact origin is debated and could be tropical or subtropical regions globally. It is widely distributed pantropically.
Distinguishing features:
Free-floating rosette of leaves, resembling a head of lettuce or cabbage.
Thick, velvety, ribbed, light green leaves with parallel veins.
Dense, feathery, fibrous roots hanging underwater.
Does not attach to the substrate; floats freely.
Reproduces rapidly, forming dense mats.
Overall appearance:
Pistia stratiotes forms a rosette of light green, velvety leaves that float on the water surface, resembling a miniature head of lettuce or cabbage. Beneath the rosette, a dense mass of feathery roots hangs submerged in the water.
Height:
The rosette of leaves typically grows 5-15 cm tall above the water, but can be larger in dense mats.
Width:
Individual rosettes range from 10-30 cm in diameter, but they quickly multiply and form extensive mats covering many square metres.
Leaves:
The leaves are thick, spongy, and have a velvety texture due to fine hairs (pubescence) on both surfaces, particularly the upper surface. They are light green, wedge-shaped (broadest at the tip and tapering towards the base), and have prominent parallel ribs or veins radiating from the centre. The margins are often wavy or scalloped. The leaves are arranged in a compact rosette.
Flowers:
The flowers are inconspicuous and unusual. They are borne on a short, fleshy stalk within the centre of the leaf rosette, hidden among the leaves. Each flower is a modified inflorescence called a spadix, surrounded by a small, pale green, leaf-like spathe (bract). Male and female flowers are separate on the same spadix, with male flowers above female flowers. Pistia stratiotes is monoecious (having separate male and female flowers on the same plant).
Flowering season:
In South Africa, it can flower and produce seeds almost continuously in warm conditions, especially from spring through autumn, whenever conditions are favourable. However, its primary mode of reproduction is vegetative (via stolons).
Fruits:
The fruit is a small, greenish berry, typically 5-10 mm long, containing several seeds. It develops inside the spathe.
Seeds:
The seeds are small, wrinkled, oblong, and light brown, about 2 mm long. They are produced within the berry, but seed production and dispersal are often less significant for its spread than its rapid vegetative reproduction.
Bark:
Not applicable, as it is a free-floating herbaceous aquatic plant.
Thorns:
This plant does not have thorns.
Wood:
Not applicable, as it is a herbaceous aquatic plant.
Roots:
It has a very dense, fibrous, feathery root system that hangs freely in the water beneath the floating rosette. These roots are efficient at absorbing nutrients from the water.
Soil type:
Not applicable, as it is a free-floating aquatic plant and does not root in soil. It thrives in nutrient-rich (eutrophic) freshwater bodies.
Aspect:
Prefers full sun to partial shade. It thrives in still or slow-moving freshwater bodies. It is sensitive to frost and salinity.
Edibility:
Water Lettuce is not considered edible for humans and should not be consumed. While some traditional uses are reported for medicinal purposes (with extreme caution), it is generally unpalatable and potentially toxic.
Toxicity:
Contains calcium oxalate crystals, which can cause irritation to the mouth, throat, and digestive system if ingested. While not highly lethal, it can cause discomfort and is generally toxic to livestock if consumed in large quantities. Its main impact is ecological.
Uses:
Historically, it was used as an ornamental aquatic plant in ponds and aquariums due to its unique appearance. In some regions, it has been used in traditional medicine or as a natural fertiliser due to its nutrient absorption. However, its invasive nature now heavily outweighs any perceived benefits.
Ecology:
As one of the world's worst aquatic alien invasive plants, Pistia stratiotes poses severe ecological and economic threats in South Africa:
Forms dense, impenetrable mats: It rapidly multiplies vegetatively, forming thick, continuous mats that can completely cover the surface of dams, rivers, canals, and other freshwater bodies.
Blocks sunlight: These mats severely reduce light penetration into the water column, shading out submerged indigenous aquatic plants and algae, leading to their death.
Depletes oxygen: The decomposition of dead plant material beneath the mats consumes large amounts of dissolved oxygen in the water, leading to anoxia (lack of oxygen). This harms fish and other aquatic invertebrates, leading to fish kills and reduced biodiversity.
Alters water chemistry: It can alter pH, temperature, and nutrient cycling within the water body.
Obstructs water flow: Dense mats impede water flow, affecting irrigation systems, hydropower generation, and increasing flood risk.
Impacts human activities: It makes recreational activities like boating, fishing, and swimming impossible. It clogs pumps and water intakes.
Reduces biodiversity: By eliminating indigenous aquatic vegetation and altering water quality, it devastates native aquatic ecosystems.
Distribution and habitat:
In South Africa, Pistia stratiotes is widespread and highly invasive in warm, freshwater bodies across most provinces, particularly in the summer rainfall regions. It is found in KwaZulu-Natal, Mpumalanga, Limpopo, Gauteng, Eastern Cape, Free State, and North West. It commonly invades dams, lakes, rivers (slow-moving sections), canals, ponds, and other impoundments. It prefers nutrient-rich waters.
Derivation of name:
The genus name Pistia is derived from the Greek word "pistis," meaning "water," referring to its aquatic habitat.
The species name stratiotes comes from the Greek "stratiotes," meaning "soldier" or "military," possibly referring to its upright, rigid leaves or its aggressive, colonising nature like an army.
Common names like "Water Lettuce" and "Water Cabbage" refer to its appearance resembling these vegetables.
Historical aspects:
Pistia stratiotes was introduced to South Africa as an ornamental plant for garden ponds and aquariums. Its rapid growth and ease of propagation made it popular. However, its ability to escape cultivation and spread rapidly in suitable aquatic environments led to its classification as a major environmental weed. It has become a significant problem in many water bodies across the country, requiring intensive management, including biological control using the Water Lettuce weevil (Neohydronomus affinis), which has had considerable success in South Africa.
Interesting facts:
This plant is like a giant, floating lettuce leaf! It drifts on the water surface and looks uncannily like a head of lettuce or cabbage.
It's a superstar at multiplying! Water Lettuce can cover an entire dam in a very short amount of time, sometimes doubling its population in just a few days. It's truly astonishing how fast it spreads.
Below the surface, it has a tangled mess of feathery roots, which are incredibly good at sucking up nutrients from the water, making the water cleaner but also causing the plant to grow even faster.
This plant is a bit of a bully. When it forms dense mats, it blocks out all the sunlight for plants underneath and sucks all the oxygen out of the water, making it a nightmare for fish and other aquatic life.
Friendly alternatives:
Common name
Scientific Name
Plant type:
Plant type
Plant family:
Araceae (Greek and Latin plant name) - Order: Alismatales