Water lettuce

Botanical Name	Pistia stratiodes
Family	Araceae
Origin	Pantropical

Identification

The plant resembles lettuce, hence the common name. The leaves are pale to bright green, 30 to 150 mm long and up to 80 mm wide. These grow from a short stem to form a densely crowded rosette. The fan-shaped leaves are ribbed towards the extremities, with conspicuous ovoid swellings of spongy cells on the upper surface. Both surfaces are covered with short depressed water-repellent hairs, which give the foliage a velvety appearance. Basal leaves in the rosette may lie flat on the water surface or cluster vertically. A dense cluster of branched roots, up to 450mm long, lies under water. The flower head, shaped like a miniature Arum lily contains small inconspicuous flowers. A cup-shaped membrane separates the male and female flowers within the flower head. The fruit are 6 to 10 mm long, oval, and have few seeds. Seeds are oblong, wrinkled, disc-like and 2 mm long.

Habitats

Water lettuce requires fertile water where temperatures approach 20°C for most of the year. It is also very frost tender.

Impact to Biota and Ecosystems

Water lettuce is one of the three most serious aquatic weeds in the world. A dense surface cover of water lettuce can restrict stock access to water, increase water loss by transpiration, and impede drainage and navigational. It can also block water supply inlets, dam waterways, cause flooding, and prevent recreational use of waterways.

Dispersal Routes, Vectors, Infestation Sources

Vegetative reproduction can be extremely rapid, when stolons (runners) from a parent stem extend horizontally beneath the water surface carrying a terminal bud which grows into a new plant. This stolon production is repeated by each daughter stem.

Management

Management
Physical Control	Control work is only to be carried out by persons authorised by the Minister of Agriculture and Fisheries. Eradication is achieved by way of hand clearance.
Disposal
Chemical Control
Biological Control
Recommended Approach

Further Comment

Water lettuce is a Notifiable Pest Plant and landowners/occupiers have a legal obligation to report the presence of this plant to AgriQuality and/or your local Pest Plants Officer who will arrange for the necessary eradication work. It is an offence for anyone to grow this plant. Notifiable Pest Plants are deemed to pose an intolerable threat to NZ and must be eradicated. The government pays for and organises all eradication work needed on this plant.

It is thought that water lettuce, a native of South America, was originally brought into New Zealand for decorative use, in 1973. It was known from only one field site in New Zealand, in thermal water at Welcome Bay, Tauranga, and it is now though to have been eradicated from all aquaria in New Zealand.

For further information on Water lettuce please refer to Environment Bay of Plenty's Fact Sheet on Aquatic Pest Plants.

This plant is prohibited from propagation, sale and distribution within New Zealand!