Home » Weed biosecurity » High priority weeds index » Water lettuce
On 1 July 2017, the Noxious Weeds Act 1993 was replaced by the Biosecurity Act 2015. The Weed biosecurity section of this website is being reviewed, and information currently on this page may not reflect the new legislation.
Young growth will cover a dam in
|Water lettuce at Wiangaree.||Mature water lettuce in a lagoon.|
Water lettuce (Pistia stratiotes) is a free-floating aquatic plant native to Asia, Africa and equatorial America. It is thought to have been introduced to NSW rivers and dams via eel traps from Queensland, and as an aquarium plant and water garden specimen sold in nurseries. Rivers, wetlands, lakes, reservoirs and slow-moving streams are most at risk from this weed, especially in the subtropical parts of the State. While Water lettuce is not established in NSW, there have been outbreaks of it throughout the northern coastal part of the State. Infestations have been located in the Tweed River catchment at Pigaben and Tyalgum, and the Richmond River catchment at Bungawalbin, Casino, Bonalbo and Grevillia. Isolated infestations have also been found at Macksville, Taree and Maitland.
At all identified sites, Water lettuce is under active control with the aim of eradication.
Water lettuce grows best on still or slow-moving bodies of fresh water, such as farm dams, reservoirs, lakes, rivers and creeks. It will tolerate temperatures between 15°C and 35°C; however optimum temperatures for growth range between 22°C and 30°C. Water lettuce is frost sensitive and growth is limited in temperate zones by long cool winters. Water lettuce can survive for long periods on mud banks or in other damp locations such as roadside culverts.
Under favourable conditions, Water lettuce will produce abundant growth, expand rapidly and form obstructive mats. These large, dense floating mats can have negative impacts on native aquatic plants and animals. They can also interfere with irrigation, boating and water sport activities. Thick mats of Water lettuce are also known to harbour disease-causing mosquitoes.
Water lettuce is a perennial plant that reproduces vegetatively and from seed. Each plant produces a number of stolons, with each producing a new rosette or daughter plant at its end. Each daughter plant will then form its own stolons, enabling the plant to increase rapidly. Once shed, the seeds will float on the water before sinking to the bottom. They germinate in early summer once temperatures rise above 20°C and then float to the surface as seedlings. Flowering and reproduction can occur as early as the four-leaf to five-leaf stage of development. When conditions for growth are good, the plant can quickly reproduce and cover an entire body of water with a thick mat of connected rosettes.
This weed is thought to have spread through the dumping of Water lettuce from aquariums or fish ponds into creeks, rivers and wetlands, or of deliberate cultivation. It is also thought to have been introduced to NSW rivers and dams via eel traps from Queensland. Water lettuce is capable of being dispersed as broken pieces, buoyant seedlings or whole plants. Pieces of Water lettuce can be spread by boats or fishing equipment moving it from an infested water body to a clean water body. Seeds can float downstream, providing a seed reserve in uninfested areas. Seeds also create ongoing problems in infested areas.
Control and management
Water lettuce is a class 1 noxious weed throughout NSW under the Noxious Weeds Act 1993. As such, the weed must be eradicated from the land and the land must be kept free of the plant.
As a notifiable weed, all outbreaks must be reported to the local council within three days.
This plant is banned from sale in NSW, ACT, Queensland, WA and NT.
Distribution map as at May 2017
Predictive mapping supplied by Queensland Biosecurity