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.

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How does this weed affect you?

Water lettuce grows quickly, blocking waterways and smothering native plants. This causes:

• poor water quality
• less fish and other water life
• blocked irrigation equipment
• problems for boating and water activities
• more habitat for mosquitos.

Rivers, wetlands, lakes, reservoirs and slow-moving streams are most affected. It is most invasive in subtropical NSW.

What does it look like?

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.  

Leaves are:

• pale green
• ribbed
• wedge-shaped
• clustered in rosettes 
• spongy to touch 
• velvety looking, with small thick hairs.

Roots are:

• unbranched 
• feathery  
• up to 60 cm long 
• floating free in the water beneath the leaves.

Flowers are:

• up to 1.5 cm long
• hidden in the centre of the plant
• whitish-green in colour
• present all year. 

Fruit are:

• green berries
• oval shaped
• 5–10 mm in diameter.

Seeds are:

• green then brown when mature
• about 2 mm long
• oblong shaped 
• in groups of 4 to 15 per berry.

Where is it found?

Water lettuce is in Queensland, NSW and Western Australia. It was once sold as an aquarium plant and for water gardens. It could have come to NSW via eel traps from Queensland. There is a debate about whether it is native in the Northern Territory, where it’s been since at least 1946. Water lettuce is native to Asia, Africa and equatorial America.

In NSW, isolated infestations are being controlled in:

• coastal areas north of Sydney 
  • Tweed River Catchment at Pigaben and Tyalgum 
  • Richmond River Catchment at Bungawalbin, Casino, Bonalbo and Grevillia.
  • Macksville
  • Taree 
  • Maitland 
• the Murray Darling Basin, in the Dumaresq River downstream of Texas. This infestation has had intense control efforts and was thought close to being eradicated. In 2016 a further infestation was found nearby. 

What type of environment does it grow in?

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.

Water lettuce can survive for long periods on muddy banks or in other damp locations such as roadside culverts.

Distribution map

• NSW (image)

How does it spread?

By seed

Flowering and seeding start from when plants develop four to five leaves. Water lettuce seeds 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.

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.

Water lettuce used to be sold at nurseries or pet shops as ‘water rose’. It’s still found in some ponds and dams. Floods can spread these plants to new areas.

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Control

Successful weed control requires follow up after the initial efforts. This means looking for and killing regrowth or new seedlings. Using a combination of control methods is usually more successful. 

To tackle 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.

Prevention

Check boating or fishing equipment for water lettuce and remove it before leaving an area.

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
• wrap in paper and dispose of it in the bin.

Physical removal

Removal by hand is effective for small infestations. Water lettuce plants cannot survive for long out of the water. To hand weed successfully:

• remove plants by raking or pulling to the bank with an encircling rope
• place all the plants above the flood line (if possible, place on plastic to prevent them from taking root)
• ensure removed plants dry out, die and break down.

Waterweed harvesters can help with larger infestations but can be expensive.

Biological control

The water lettuce weevil Neohydronomus affinis causes considerable damage and can control water lettuce within several years in tropical and subtropical regions. Adult weevils are 1.8 mm long and they make tiny holes in the leaves.

This weevil is available for further distribution. Please contact your weeds officer if you would like to use biocontrol to control water lettuce.

Chemical control

Herbicide can help to control large infestations of water lettuce.

Herbicide options

See Using herbicides for more information.

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PERMIT 83083 Expires 31/03/2022
Carfentrazone-ethyl 240 g/L (Shark™ Aquatic Herbicide)
Rate: 933 mL of product per ha
Comments: DO NOT apply more than two (2) applications per year with a minimum re-treatment interval of 90 days between consecutive treatments.
Withholding period: Nil.
Herbicide group: G, Inhibitors of protoporphyrinogen oxidase (PPOs)
Resistance risk: Moderate

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PERMIT 83083 Expires 31/03/2022
Carfentrazone-ethyl 240 g/L (Shark™ Aquatic Herbicide)
Rate: 933 mL of product per ha
Comments: DO NOT apply more than two (2) applications per year with a minimum re-treatment interval of 90 days between consecutive treatments.
Withholding period: Nil.
Herbicide group: G, Inhibitors of protoporphyrinogen oxidase (PPOs)
Resistance risk: Moderate

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PERMIT 84767 Expires 31/05/2020
Metsulfuron-methyl 600 g/kg (Brush-off®)
Rate: 10 g per 100 L water plus a wetter at 200 mL per 100 L water
Comments: May only be applied in enclosed water bodies, and not within 400 m of potable water supply uptakes. WARNING: very toxic to aquatic plants and algae. Apply a maximum of 3 applications per year at minimum intervals of 90 days.
Withholding period: Nil (recommended not to graze for 7 days before treatment and for 7 days after treatment to allow adequate chemical uptake in target weeds).
Herbicide group: B, Inhibitors of acetolactate synthase (ALS inhibitors)
Resistance risk: High

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2,4-D 300 g/L (Affray 300®)
Rate: 1.0 L in 200 L of water
Comments: Avoid causing submersion of sprayed plants. Coverage: 200 L spray solution per 1000 square metres.
Withholding period: 7 days.
Herbicide group: I, Disruptors of plant cell growth (synthetic auxins)
Resistance risk: Moderate

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Diquat 200 g/L (Reglone®)
Rate: 400 mL per 100 L of water
Comments: Add Agral 600 wetter, use clean water for best results. Observe withholding period.
Withholding period: 1 day in pasture, 10 days in treated water.
Herbicide group: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
Resistance risk: Moderate

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Diquat 200 g/L (Reglone®)
Rate: 5.0–10.0 L/ha
Comments: Add Agral 600 wetter, use clean water for best results. Observe withholding period.
Withholding period: 1 day in pasture, 10 days in treated water.
Herbicide group: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
Resistance risk: Moderate

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Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: 1.0–1.3 L in 100 L of water
Comments: Best results are obtained from mid-summer through to winter. Use higher rate on dense infestations.
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area	Duty
All of NSW	General Biosecurity Duty All plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
Greater Sydney	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
Hunter	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
Murray	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
North Coast Exclusion zone: whole region except for the core infestation area of Swan Bay on the Richmond River	Regional Recommended Measure* Whole region: The plant or parts of the plant should not be traded, carried, grown or released into the environment. Exclusion zone: The plant should be eradicated from the land and the land kept free of the plant. Land managers should mitigate the risk of the plant being introduced to their land. Core infestation area: Land managers should reduce impacts from the plant on priority assets.
North West	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment.
Northern Tablelands	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
Riverina	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
South East	Regional Recommended Measure* Land managers should mitigate the risk of new weeds being introduced to their land. The plant should be eradicated from the land and the land kept free of the plant. The plant should not be bought, sold, grown, carried or released into the environment. Notify local control authority if found.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfill the general biosecurity duty for this weed click here

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Reviewed 2020