08 Nov Water lettuce
Pistia stratiotes
Water lettuce is a free floating weed with pale green leaves. It spreads rapidly and forms dense mats over water bodies.
Family: Araceae
Origin: Water lettuce is native to Asia, Africa and equatorial America.
Habit: Found in rivers, wetlands, lakes, reservoirs and slow-moving streams. It is most invasive in subtropical NSW. Water lettuce grows quickly, blocking waterways and smothering native plants. It causes poor water quality, reduces food and habitat for fish and other water life, blocks irrigation equipment, causes problems for boating, fishing and water activities and provided habitat for mosquitos.
Water lettuce looks like an open head of lettuce floating on the water. Its roots hang in the water and do not attach to the bottom. Plants are up to 15 cm high and 30 cm wide. Plants produce daughter plants that remain attached to the parent plant by white root-like stolons. These can be 60 cm long.
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 ranges between 22°C and 30°C. Water lettuce is frost sensitive and growth is limited in temperate zones by long cool winters. It can survive for long periods on muddy banks or in other damp locations such as roadside culverts.
Leaves: Its leaves are pale green and velvety looking, with small thick hairs, 2–17 cm long and 1.5–8.5 cm wide, ribbed and spongy, wedge-shaped, clustered in rosettes on short inflated stalks.
Flowers: Its flowers are up to 1.5 cm long, hidden in the centre of the plant, whitish-green in colour and present all year.
Fruit: and Seeds: It fruit are oval shaped green berries about 5–10 mm in diameter. Its seeds are oblong shaped green then brown when mature, about 2 mm long in groups of 4 to 15 per berry.
Roots: Its roots are unbranched, feathery, up to 60 cm long, floating free in the water beneath the leaves.
Dispersal: It was once sold as an aquarium plant and for water gardens. It could have come to NSW via eel traps from Queensland.
Dispersal by seed
The plants start flowering and seeding after they have four to five leaves, they float on the water for a while, before sinking to the bottom. This helps the plant spread downstream to new areas. The seeds germinate in early summer once temperatures rise above 20°C. Then the new seedlings float to the surface. The buoyant seedlings can move further downstream. Seeds create ongoing problems in infested areas.
Dispersal by plant parts
Water lettuce grows all year round. Each plant produces daughter plants. Each daughter plant then produces its own daughter plants, forming dense mats. When broken up, each plant, or even pieces of plants, can start to grow a new mat.
Boats and fishing equipment can move water weeds into clean water bodies. Water and plant fragments from fish tanks or garden ponds can spread the weed to new areas. Floods can also spread these plants to new areas.
Control: Successful weed control relies on follow up after the initial efforts. This means regularly looking for any new plants or plant fragments and controlling them. Using a combination of control methods is usually more successful.
To manage water lettuce:
> notify your local council if you see an infestation so they can help control it
> take care to prevent the spread of plant parts
> physically remove plants from the water
> consider herbicide treatments to help manage large infestations
> follow up with physical removal of any surviving plants or regrowth.
Water Lettuce
Preventing this weed establishing is much better than having to control it. So do not dispose of unwanted fish tank or garden pond plants into dams, creeks or streams. When removing plants from a fish tank or garden pond, remove from the water, let plants dry out completely and then wrap in paper and dispose of it in the bin.
