Water lettuce (Pistia stratiotes)

Also known as: Nile cabbage, dwarf water lettuce

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. It causes:

• poor water quality
• less food and habitat for fish and other water life
• blocked irrigation equipment
• problems for boating, fishing 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 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.

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.

Maps and records

• Recorded presence of Water lettuce during property inspections (Map: Biosecurity Information System - Weeds, 2017-2023)
  These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.
  
  

How does it spread?

By seed

The plants start flowering and seeding after they have 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.

References

Hosking, JR, Sainty, G, Jacobs, S and Dellow, J (in prep.) The Australian WEEDbook.

Parsons, W. T., Parsons, W. T., & Cuthbertson, E. G. (2001). Noxious weeds of Australia. CSIRO publishing.

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 12 February 2020 from https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Pistia~stratiotes

Sainty, G.R. and Jacobs, S.W.L. (2003). Waterplants in Australia 4th ed. Sainty and Associates, Darlinghurst. 

Trounce, B. (2004). Aquatic weed control in small dams and waterways. Agfact P7.2.1 (4th ed.)

More information

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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.

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 92971 Expires 31/03/2028
Metsulfuron-methyl 600 g/kg (LYNX WG herbicide)
Rate: 10 g / 100 L water (add wetter at 200 mL per 100 L)
Comments: Only for enclosed water bodies. Areas within 400 m of potable water supply uptakes are excluded. ONLY apply as a foliar spot spray during non-frost periods using a high volume sprayer/knapsack fitted with calibrated spray equipment. Apply a maximum of 3 applications per year at minimum intervals of 90 days. WARNING: Very toxic to aquatic plants and algae. See permit for more restraints and critical use comments.
Withholding period: A nil withholding period is applicable. However, it is recommended not to graze treated areas for 3 days to ensure product efficacy.
Herbicide group: 2 (previously group B), Inhibition of acetolactate and/or acetohydroxyacid synthase (ALS, AHAS inhibitors)
Resistance risk: High

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PERMIT 83083 Expires 30/04/2025
Carfentrazone-ethyl 240 g/L (Shark™ Aquatic Herbicide)
Rate: 933 mL of product per ha
Comments: For nonflowing water bodies. DO NOT apply more than two applications per year with a minimum re-treatment interval of 90 days between consecutive treatments. See permit for further critical comments.
Withholding period: Not required when used as directed.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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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. For non-potable water. See label for other restraints.
Withholding period: Do not graze or cut for stock food for 7 days after application.
Herbicide group: 4 (previously group I), Disruptors of plant cell growth (Auxin mimics)
Resistance risk: Moderate

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Diquat 200 g/L (Reglone®)
Rate: 400 mL per 100 L of water
Comments: Add 150 ml Agral 600 wetter/ 100 L of water. For small areas: Spray to wet weeds thoroughly. About 1 mL of product should be sufficient for 1 square metre of weeds.
Withholding period: Do not use treated water for human consumption, livestock watering or irrigation purposes for 10 days after application. Do not graze or cut sprayed vegetation for stock food for 1 day after application. See label for harvest withholding periods.
Herbicide group: 22 (previously group L), Inhibition of photosynthesis at photosystem I via electron diversion (PSI electron diversion)
Resistance risk: Moderate

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Diquat 200 g/L (Reglone®)
Rate: 5.0–10.0 L/ha
Comments: Apply as overall spray wetting foliage thoroughly. Use higher rates for heavy infestations or for deep or dirty water. Oxygen depletion of decaying weeds may occur therefore do not spray more than a quarter of the area at any one time.
Withholding period: Do not use treated water for human consumption, livestock watering or irrigation purposes for 10 days after application. Do not graze or cut sprayed vegetation for stock food for 1 day after application. See label for harvest withholding periods.
Herbicide group: 22 (previously group L), Inhibition of photosynthesis at photosystem I via electron diversion (PSI electron diversion)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: 1 tablet for every 37.5 cubic metres of water to achieve 400 parts per billion.
Comments: For use on dense or established weed populations in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger, still water bodies. Throw tablets directly into the water to achieve uniform distribution of the herbicide. Alternatively, if weeds grow in clusters, concentrate the tablet application on the densest areas. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: 1 tablet for every 75 cubic metres of water to achieve 200 parts per billion.
Comments: For use on low density, establishing or re-establishing weeds in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger, still water bodies. Throw tablets directly into the water to achieve uniform distribution of the herbicide. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: Spray 12 - 15 L of solution per 100 m2. Solution = 1 tablet per 50 L water + 0.5 to 1.0% adjuvant/surfactant.
Comments: Spray on dense or established weed populations in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger, still water bodies. See label for restrictions
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: Spray 12 - 15 L of solution per 100 m2. Solution = 1 tablet per 100 L water + 0.5 -1.0% adjuvant/surfactant.
Comments: Spray on low density, establishing or re-establishing weeds in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger still water bodies. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO 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: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
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 pest 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* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
Hunter	Regional Recommended Measure* (for Regional Priority - Eradication) Notify local control authority if found. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant.
Murray	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
North Coast	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
North West	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
Northern Tablelands	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
Riverina	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found. Your local biosecurity weeds officer can help to identify, advise on control, and how to remove this weed.
South East	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
Western	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfil the general biosecurity duty for this weed click here

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Reviewed 2023