Water lettuce (Pistia stratiotes)

WEED ALERT: STATE PROHIBITED WEED
If you see this plant contact your council weeds officer, the NSW Invasive Plants & Animals Enquiry Line 1800 680 244 or email weeds@dpi.nsw.gov.au

Profile

Impact

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.

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’.

Description

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.

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 Department of Environment and Conservation (DEC) for any herbicide applications applied over or near water.

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.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.


PERMIT 14447 Expires 31/03/2017
Metsulfuron-methyl 600 g/kg (Brush-off®)
Rate: 10 g in 100 L of water
Comments: Handgun application, add wetter, synertrol oil at 200 mL in 100 L of water, avoid broadcasting spray over the water
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


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


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


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


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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Legal requirements

The content provided here is for information purposes only and is taken from the Noxious Weeds (Weed Control) Order 2014 published in the NSW Government Gazette, detailing weeds declared noxious in New South Wales, Australia, under the Noxious Weeds Act 1993. The Order lists the weed names, the control class and the control requirements for each species declared in a Local Control Authority area.

Area Class Legal requirements
All of NSW 1 State Prohibited Weed
The plant must be eradicated from the land and that land must be kept free of the plant

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Water lettuce leaves have a velvety appearance.
Water lettuce leaves have a velvety appearance. (Photo: Brian Worboys)

Water lettuce forms dense mats over the water surface.
Water lettuce forms dense mats over the water surface. (Photo: Charlie Mifsud)

Water lettuce plants resemble open heads of lettuce.
Water lettuce plants resemble open heads of lettuce. (Photo: Brian Worboys)

Water lettuce can form a dense mat over an entire body of water.
Water lettuce can form a dense mat over an entire body of water. (Photo: Brian Worboys)

Whitish-green flowers are hidden in the centre of the plant at the base of the ribbed leaves.
Whitish-green flowers are hidden in the centre of the plant at the base of the ribbed leaves. (Photo: Charlie Mifsud)

Water lettuce invades slow moving fresh water.
Water lettuce invades slow moving fresh water. (Photo: Andrew Petroeschevsky)

Water lettuce is free floating with submerged feathery roots.
Water lettuce is free floating with submerged feathery roots. (Photo: Rod Ensbey)