Water lettuce (Pistia stratiotes)

Water lettuce is a free floating, aquatic weed which can spread rapidly forming dense mats over entire bodies of water.

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How does this weed affect you?

Water lettuce is a free floating aquatic plant with the potential to quickly spread and form a dense mat that can cover an entire body of water.

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.

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.

Where is it found?

Water lettuce is native to Asia, Africa and equatorial America. There is debate about whether the Northern Territory, where it was collected in 1946, forms part of its native range. It has since spread to Queensland, NSW and Western Australia.

Isolated infestations of water lettuce have occurred in coastal districts of New South Wales (NSW) north of Sydney - 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. Some of these infestations are thought to have been introduced into NSW on eel trapping equipment. There is also an isolated infestation in the NSW portion of the Murray Darling Basin, in the Dumaresq River downstream of Texas. This infestation, which is thought to have spread from upstream, has been under intense control and is close to being eradicated.

Distribution map

• NSW (image)

How does it spread?

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- 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 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 to a clean water body.

Seeds can float downstream providing a seed reserve in uninfested areas. Seeds also create ongoing problems in infested areas.

Other methods of dispersal are thought to include dumping of aquarium or fish pond water into creeks and rivers, contamination of boats and fishing equipment, and even deliberate cultivation, it is sometimes still being sold in nurseries or pet shops often labeled as ‘water rose’.

What does it look like?

Water lettuce is a free floating plant that looks like an open head of lettuce. It can grow up to 15 cm tall and 30 cm wide. Mother and daughter plants are attached by stolons (white root-like structures which link plants together) up to 60 cm long.

Leaves

Pale green leaves are ribbed, wedge-shaped and form a rosette. They are spongy to touch and have a velvety appearance due to the small thick hairs that cover them.

Roots

A large number of unbranched feathery roots up to 80 cm long are submersed in water beneath the leaves of the plant.

Flowers

The flowers of water lettuce are very small (up to 1.5 cm long) and hidden in the centre of the plant amongst the leaf bases. They are whitish-green in colour. Flowering occurs throughout the year.

Fruit

The fruit is a greenish berry, 5–10 mm in diameter.Four to fifteen oblong shaped seeds occur in each berry. They are green at first then mature to a brown colour and are about 2 mm long.

Acknowledgements

Authors: Rachele Osmond, Stephen Johnson. Prepared by: Kirrily Condon; Reviewed by Peter Gray & Charles Mifsud; Edited by Elissa van Oosterhout, Birgitte Verbeek

References

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

Ensbey, R (2004) Noxious and environmental weed control 2004–2005, a guide to weed control in non-crop, aquatic and bushland situations. NSW Agriculture.

Hay, A (1993) Araceae. Family 168. In, Flora of New South Wales, Volume 4. Ed. GJ Harden. University of New South Wales Press, Kensington. Pp. 31-36.

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

Parsons, WT and Cuthbertson. EG (1992) Noxious Weeds of Australia. Inkata Press. Pp. 38-41.

Sainty, GR and Jacobs, SWL (2003) Waterplants in Australia 4th ed. Sainty and Associates, Darlinghurst. pp. 10-11

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Control

Preventing spread

If you find water lettuce, notify the local council so that appropriate control methods can be carried out.

Emptying unwanted aquarium or fish pond plants into dams, creeks or streams is an offence and may harm the environment. When removing plants from an aquarium, remove from the water and let the plant dry out completely before wrapping in paper and disposing of it in the bin.

If you suspect anyone has water lettuce and they are not aware of the problems it poses, you should advise them to contact the local council weeds officer for advice. If using an area for recreational water sports, prior to leaving, check equipment for water lettuce and remove it.

While water lettuce should no longer be available for sale in NSW, it still does appear in nurseries, pet shops and on the internet, often being sold as ‘water rose’. If you notice it being sold, report the instance to the local council weeds officer.

Physical removal

Physical removal is effective for small infestations. Water lettuce plants cannot survive for long out of the water and can be removed by either raking or being pulled to the bank with an encircling rope.

Once removed, plants must be allowed to dry out and break down. Make sure that all plants removed are placed above the floodline. If possible, place on plastic to prevent them from taking root in mud.

Water weed harvesting craft may be suitable for larger infestations although these can be quite expensive.

Biological control

Insects such as the weevil Neohydronomus affinus have been introduced for biological control in Australia. 

While this insect has not been released in NSW, it has shown to be effective on dams in south-eastern Queensland.

Chemical control

Herbicide may be necessary to control large infestations of water lettuce. When applying herbicides to a water body, it is important to follow these procedures:

• Identify the plant or plants correctly.
• Select a chemical registered for use in water and on that particular plant.
• Read the chemical label carefully and observe all special precautions.

A permit may need to be obtained from the relevant government department for any herbicide applications applied over or near water.

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 2018