Water soldier (Stratiotes aloides)

Water soldier is a water weed that can grow above the water in summer but is submerged for the rest of the year. It can overrun waterways, displacing other plants and animals.

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How does this weed affect you?

Water soldier forms dense stands in waterways. It:

• outcompetes native aquatic plants
• reduces food and habitat for fish and other aquatic animals
• secretes chemicals that kill plankton 
• makes boating and recreational activities difficult. 

What does it look like?

Water soldier is a perennial plant that can grow in water up to 5 m deep. For most of the year it grows under water. In spring the new leaves contain air pockets which help the plant float. In summer the plant flowers above the water. As older leaves die back in autumn they become waterlogged which causes the plant to sink again.

Water soldier can remain submerged all year round if there is not enough sunlight reaching the underwater plants.

Leaves are:

• a large rosette that looks a bit like a pineapple top or aloe
• stalkless and slightly clasping the stem.

Leaves above water are:

• dark green
• fleshy
• usually less than 40 cm long, 1–4 cm wide
• serrated with spines along the margins.

Underwater leaves are:

• lighter green or reddish-purple 
• thinner than above water leaves, up to 60 cm in length, and usually 1–2 cm in wide
• sometimes wavy along the edges with weak spines.

Flowers:

The flowers smell foul. Female and male flowers are usually on separate plants. 

Female flowers:

• have 3 white roundish petals
• have 3 green sepals (outer leaf-like structures surrounding the petals) with purplish stripes
• are on a thick stalk up to 30 cm long
• are usually solitary but sometimes in pairs
• have two modified leaves (bracts) below the flowers that are turned inwards at the tip and look like a crab claw.

 Male  flowers:

• have 3 white roundish petals and translucent sepals
• are on a flattened stalk up to 30 cm long
• are in groups of 2 to 6 flowers 
• have roundish bracts that point outwards.

Fruit are:

• a berry-like capsule
• brownish-green to green
• 1–3 cm long
• egg or barrel-shaped with a cone-shaped tip
• thick and leathery
• filled with pulp containing 12–24 seeds.

Seeds are:

• cylindrical
• often curved at the end with a pronounced beak
• up to 11 mm long and 3 mm wide

Roots are:

• simple and covered with long, silky fibres
• up to 1.8 m long
• hang free in the water or loosely rooted in the substrate.

Where is it found?

Water soldier does not currently grow in Australia. If it was introduced to NSW,  it could invade freshwater ponds, lakes and streams. 

It is native to Europe and central Asia.

What type of environment does it grow in?

Water soldier grows in temperate regions. Plants can grow in still or slow-moving fresh water up to 5 m deep but do not grow in water if the levels fluctuate.  It tolerates water with a wide variety of pH values.

Maps and records

• Recorded presence of Water soldier during property inspections (Map: Biosecurity Information System - Weeds, 2017-2024)
  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

Seeds are released from the fruit in a pulp that sinks to the bottom. The seeds eventually float downstream. Seed production can be limited when male and female plants are not growing close to each other. 

By plant parts

In summer, stolons produced from the base of the parent plant break away to form new rosettes. Up to 10 plants can form around each parent plant.

References

Cook, C. D., & Urmi-König, K. (1983). A revision of the genus Stratiotes (Hydrocharitaceae). Aquatic botany, 16(3), 213-249.

Efremov, A. N., Sviridenko, B. F., Toma, C., Mesterházy, A., & Murashko, Y. A. (2019). Ecology of Stratiotes aloides L.(Hydrocharitaceae) in Eurasia. Flora, 253, 116-126.

Government of South Australian (2014).  Declared Plant Policy under the Natural Resources Management Act 2004 water soldier (Stratiotes aloides).  Retrieved 30 July 2020 from: https://pir.sa.gov.au/__data/assets/pdf_file/0016/137320/water_soldier_policy.pdf

Harpenslager, S. F., Smolders, A. J., Kieskamp, A. A., Roelofs, J. G., & Lamers, L. P. (2015). To float or not to float: how interactions between light and dissolved inorganic carbon species determine the buoyancy of Stratiotes aloides. PLoS One, 10(4), e0124026.

Mulderij, G., Mooij, W. M., Smolders, A. J. P., & Van Donk, E. (2005). Allelopathic inhibition of phytoplankton by exudates from Stratiotes aloides. Aquatic Botany, 82(4), 284-296.

Sainty, G. R., & Jacobs, S. W. (2003). Waterplants in Australia (No. Ed. 4). Sainty and Associates Pty Ltd.

Snyder, E., Francis, A., & Darbyshire, S. J. (2016). Biology of invasive alien plants in Canada. 13. Stratiotes aloides L. Canadian Journal of Plant Science, 96(2), 225-242.

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Control

Please do not attempt to treat or dispose of this weed yourself. Report this plant if you see it anywhere in NSW by calling the helpline listed at the top of this page immediately. 

NSW DPI will lead an initial response for the treatment and disposal of the plant to stop it from spreading.

Herbicide options

See Using herbicides for more information.

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: One part product to 50 parts water
Comments: Spot spray
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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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: One part product to 9 parts water
Comments: Splatter gun
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.
All of NSW	Prohibited Matter A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries

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Reviewed 2020