Water lettuce - Pistia stratiotes

Department of Agriculture, Fisheries and Forestry 

Biosecurity Queensland 

Water lettuce 

Pistia stratiotes 

Fact sheet 

DECLARED CLASS 2 PEST PLANT 

PP19 December 2013 

Water lettuce 

Pistia stratiotes 

Fact sheet 

DECLARED CLASS 2 PEST PLANT 

Water lettuce is a free-floating aquatic weed that rapidly 

forms dense mats covering rivers, dams and irrigation 

canals. It can restrict water flow, increase water loss by 

evapotranspiration and can serve as a breeding ground 

for mosquitoes. I creates a desne cover on the water 

surface that prevents light penetration and exchange of 

gasses. This leads to a drop of oxygen in the water whish 

is detrimental to the aquatic ecosystem. Water lettuce 

spreads both by vegetative reproduction and by seeds. 

There is a big infestation in the Warrego River at 

Cunnamulla that is of concern, as much of the Murray– 

Darling catchment is currently free of this weed and 

all efforts should be made to control this infestation 

and prevent further introduction. In all other areas of 

Queensland, an integrated control strategy should be 

adopted to reduce infestations. 

Declaration details 

Water lettuce is a declared Class 2 plant under Land 

Protection (Pest and Stock Route Management) Act 2002. 

A Class 2 pest is one that has already spread over 

substantial areas of Queensland, but its impact is so 

serious that we need to try and control it and avoid 

further spread onto properties that are still free of the 

pest. By law, all landholders must try to keep their land 

free of Class 2 pests and it is an offence to keep or sell 

these pests without a permit. A local government may 

serve a notice upon a landholder requiring control of 

declared pests. 

Great state. Great opportunity.

Description and general information 

As the name suggests, the entire plant resembles a small, 

floating, open head of lettuce. The fan-shaped leaves are 

pale green, have marked parallel veins, are spongy and 

covered with hairs and form a floating rosette. The spongy 

tissue and the air that gets trapped by the hairs give the 

plant its buoyancy. It has a tuft of unbranched, fibrous 

roots up to 80 cm long. 

Mature plants produce a large number of small, pale white 

to green, inconspicuous flowers hidden at the base of the 

leaves of the rosette. A membrane separates the male and 

female flowers. The fruit is a berry 5–8 cm across with 

oblong seeds about 2 mm long, tapered at each end. 

The problem 

Under tropical conditions, growth is rapid; dense mats 

form, often covering rivers, dams and irrigation canals. 

These mats, with their long roots, can restrict the flow of 

water, increase water loss through evapotranspiration 

and reduce light penetration that is necessary for 

photosynthesis in submerged aquatic plant life. The lack 

of oxygen and the shading significantly impact on native 

aquatic animals and plants. 

The presence of water lettuce is especially undesirable in 

the Murray–Darling system as it could possibly spread into 

inland New South Wales, Victoria and South Australia were 

it is currently not established. 

Heavy infestations can affect waterbodies in a number 

of ways: 

Safety and health risk 

Children and livestock may be in danger of drowning if 

they become entangled in the roots of a heavy infestation. 

Water lettuce mats may also create a haven for mosquitoes 

that are vectors of Ross River fever and encephalitis. 

Interference with irrigation and stock watering 

Stock may have difficulty gaining access to drinking water 

if the surface is completely covered by weeds. 

Water flow to irrigation equipment is reduced due to the 

restrictive action of the roots, which in turn increases 

pumping times and costs. 

Damage to structures 

Under flood conditions, rafts of plant material build up 

at fences and bridges that in turn collect other floating 

debris. The combined weight may cause these structures 

to collapse. 

Loss of water 

High rates of evapotranspiration through the leaves can 

increase water loss during summer. 

Degradation of water quality 

Heavy water lettuce infestations reduce the penetration 

of sunlight necessary for native plant growth in water 

bodies preventing aquatic photosynthesis and thereby 

reduce the availability of oxygen to other organisms. 

Heavy weed cover also prevents the exchange of air, which 

normally occurs on an open water surface, further reducing 

oxygenation of the water. 

As the plant dies and decomposes, oxygen is removed 

from the water; this causes water pollution and 

stagnation. This stagnation affects water quality and may 

result in the death of aquatic animals. 

Destruction of wildlife habitats 

A large infestation of water lettuce is a physical barrier 

for aquatic and semi-aquatic animals, restricting their 

territorial movements and breeding activities. 

Recreation and aesthetics 

Large infestations of water lettuce stop the passage of 

boats by clogging the inlets of boat engine 

water-cooling systems. The mats of weed also interfere 

with swimming and make fishing impossible. The natural 

beauty of an open waterbody can be spoilt and further 

degraded as native aquatic plants, birds and animals 

are displaced. 

Life cycle 

Active growth of plants begins with increasing 

temperatures in spring. Flowering commences early in the 

plant’s life–about the fourth or fifth leaf stage; high plant 

densitities trigger flowering. After flowering, seeds drop to 

the muddy bottom and form a persisting seed bank. The 

plants can produce large number of seeds over a season 

(several thousands per meter square). Seeds germinate in 

late November to early December the next season and float 

to the surface as seedlings. 

Plants reproduce vegetatively throughout the growth 

season. Each mother plants generates several daughter 

plants that row on stolons (up to 20 cm long). Water 

lettuce is sensitive to frost. 

Habitat and distribution 

The origin of water lettuce remains unclea and was 

introduced to Australia as an aquarium and water garden 

specimen. As a result of flooding and abandonment, 

the plant is now found in a number of areas throughout 

eastern Queensland. The plant prefers stationary or 

slow-moving streams and could become established in 

most areas of the state. Water lettuce thrives in water with 

high nutrient concentrations. 

Like other floating plants, water lettuce can survive for 

long periods when stranded on mud banks or in other 

damp situations (such as a roadside culvert where it may 

even take root). 

2 Water lettuce Pistia stratiotes

Map 1 Distribution of water lettuce in Queensland 

Mechanical control of water lettuce can help take 

advantage of flooding or water flushes that deposit water 

lettuce in dams, lagoons and calm water areas of rivers 

and creeks. When using this approach it is essential water 

lettuce is removed before its rapid growth commences. 

Water lettuce can survive if it is deposited or left on moist 

banks it. To help prevent the reintroduction of water 

lettuce into the watercourse, it is essential it is moved 

from the waters edge. 

Biological control 

Two weevils (Neohydromonus pulchellus and Orchetina 

bruchi) have been introduced and exert good control in 

some areas. 

The weevils have proved effective on dams from 

Bundaberg to Brisbane. The life cycle of the weevil takes 

about three months. Eggs are laid in the fleshy leaves and 

the larvae tunnel through the plant tissue. The openings 

assist the entry of fungi and bacteria, causing the tissue 

to rot. 

Biological control is most effective on large infestations, 

but it may take several years to achieve satisfactory 

control if used in isolation from other methods. To 

establish an effective breeding population of biological 

control agents, infested plants should be placed in an area 

where the water lettuce is concentrated. 

Your local government office or local Biosecurity 

Queensland officer can assist you with protocols and 

information on the collection site nearest to you. 

Prevention 

New infestations may become established when plants 

are brought into new areas by flooding or accidential 

transport, i.e. through boating. Surveillance and physical 

removal at such times may prevent infestation. Public 

education to prevent disposal of plants from domestic 

situations is necessary. 

Control 

In most cases the best management approach combines 

herbicide, mechanical, fire and biological control methods 

with land management changes. It is essential to choose 

control methods that suit the specific weed and the 

particular situation. 

Mechanical control 

Water lettuce removal by hand or racking is a practical 

control method often used for small areas or when 

numbers are low. The use of water weed harvesters may 

also be used in larger areas. 

Physical removal is most effective for small infestations 

and should be made before flowering and seeding. 

Water lettuce Pistia stratiotes 3

Herbicide control 

Before using any herbicide always read the label carefully. 

Apply all herbicides strictly in accordance with the 

directions on the label. When treating water which is used 

for irrigation purposes, the withholding period should be 

followed in accordance with the label recommendations. 

Spraying with herbicides is often the only practical method 

for large infestations. Autumn applications tend to be more 

effective than summer ones. 

Table 1 lists the herbicides registered for control of 

water lettuce. 

Integrated control 

Integrated control is a sensible strategy that includes 

a combination of mechanical, biological and herbicide 

methods that complement each other. 

First make certain that the weevils are established 

on the infestation, and then carry out mechanical 

control or a spray program using a selective herbicide. 

Selectively controlling strips of the water lettuce mats 

helps concentrate the biological control insects onto the 

remaining weed to increase damage. 

Mechanical removal of dead plants will avoid water quality 

degradation by masses of rotting weed. 

Further information 

Further information is available from your local 

government office, or by contacting Biosecurity 

Queensland (call 13 25 23 or visit our website at 

www.biosecurity.qld.gov.au). 

Table 1 Herbicides registered for the control of water lettuce 

1, 2, 3 

Situation Herbicide Rate Comments 

Waterways, non potable 

water, drains, dams, 

margins lakes and 

streams 

2,4-D acid 

(e.g. AFFRAY 300) 

2,4-D acid 

(e.g. AFFRAY 300) 

50 mL/10 L water 

or 500 mL/100 L water 

Knapsack: coverage 10 L spray solution/100 m² 

Avoid causing submersion of sprayed plants 

5 L/200 Lwater/ha Boom application by helicopter. Raindrop D8 

nozzles angled back at 45 degrees to minimise 

spray drift 

Aquatic areas 

Diquat (20 g/L) 

e.g. Watrol non-residual 

herbicide® 

50 L to 100 L/ha 

or 4 L/100 L water 

Add wetting agent. Higher rate for denser 

infestations or dirty water. Overall spray, 

thoroughly wetting foliage. Follow label 

instructions 

Add wetting agent. Follow label instructions 

Aquatic areas (drains, 

channels, margins of 

streams, lakes and 

dams) 

Aquatic areas 


e.g. Vegetrol herbicide ® 


e.g. Vegetrol herbicide ® 

Calcium dodecyl 

benezene sulphonate 

(e.g. immerse) 

50 L/ha to 100 L/ha 

or 4 L/100 L water 

5 L/ha to 10 L/ha 

or 0.4 L/100 L water 

1:19 with kerosene. 

Apply 1 L of mixture per 

100 m² 

Add wetting agent. Follow label instructions 

Sprinkle onto free floating plants and adjacent 

water surface lightly, just enough to change their 

normal colour. Don’t spray dense solid mats with 

no visible water surface. 

Use in Summer. Follow label instructions 

Glyphosate (360 g/L) 

(numerous) 

1 L to 1.3 L/ 100 L water 

or 6 L/ha to 9 L/ha 

Notes: 

Diquat is the only product registered for use in water storage areas used for human consumption. Note that 14 days must 

elapse after treatment before water can be consumed. 

Read the label carefully before use. Always use the herbicide in accordance with the directions on the label. 

This fact sheet is developed with funding support from the Land Protection Fund. 

Fact sheets are available from Department of Agriculture, Fisheries and Forestry (DAFF) service centres and our Customer Service Centre (telephone 13 25 23). Check 

our website at www.biosecurity.qld.gov.au to ensure you have the latest version of this fact sheet. The control methods referred to in this fact sheet should be used in 

accordance with the restrictions (federal and state legislation, and local government laws) directly or indirectly related to each control method. These restrictions may 

prevent the use of one or more of the methods referred to, depending on individual circumstances. While every care is taken to ensure the accuracy of this information, 

DAFF does not invite reliance upon it, nor accept responsibility for any loss or damage caused by actions based on it. 

© The State of Queensland, Department of Agriculture, Fisheries and Forestry, 2013.

Water lettuce - Pistia stratiotes

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