Water caltrop (Trapa species)

Also known as: floating water chestnut

Water caltrop is a water weed that forms dense mats blocking waterways. Its leaves can float or grow under the water and it produces nuts with sharp spines.

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How does this weed affect you?

Water caltrop:

• can form thick impenetrable mats across wide areas of water 
• blocks access to water 
• has sharp spines on the nuts that can hurt humans and animals
• outcompetes native plants
• reduces food and habitat for fish and other aquatic animals
• prevents recreational activities such as swimming and fishing.

What does it look like?

Water caltrop is an annual water weed. It dies over winter and then grows back from seeds in spring.

There are six species in the Trapa genus.  All Trapa species are prohibited matter in NSW. The following description is based on Trapa natans which is the most likely species to be invasive. This species includes two variations that are both commonly called water caltrop: T. natans var. natans  and  T. natans var. bispinosa.

Leaves

Leaves are either submerged or floating.

Submerged leaves are:

• feather-like
• arranged in whorls around the stem.

Surface leaves are:

• in circular clusters (rosettes), with leaves radiating out
• glossy on top
• 2–3 cm long
• oval, triangle or diamond-shaped with saw-toothed edges
• on stalks that usually have a swollen round bulge
• covered in fine, short hairs underneath.

Flowers are:

• white with four petals
• 8 mm long 
• above the water's surface
• present in early summer.

Fruit are:

• a hard, woody or bony nut with sharp spines
• about 3 cm wide
• filled with one seed
• under the floating leaves. 

Trapa natans var. natans has 4 spines and Trapa natans var. bispinosa has two spines.

Stems are:

• submerged
• long and unbranched reaching 3.6–4.5 m in length
• anchored into the mud by very fine roots.

Roots are:

• up to 8 cm long
• often mistaken for feather-like leaves.

Submerged leaves drop off during early stem growth and roots form at the points were the leaves dropped off.

Similar looking plants

Water caltrop looks similar to:

• mosaic flower (Ludwigia sedioides), which is sometimes grown in ponds and water features. It has yellow flowers about 3 cm wide and doesn’t have inflated leaf stalks.

Where is it found?

Water caltrop is not known to occur in Australia. There is archival evidence that it was grown in Brisbane by the Queensland Acclimatisation Society in the late 1800s.

It is native to the warm temperate parts of Eurasia and Africa. It has become invasive in eastern areas of Canada and the United States. Water caltrop is also cultivated around the world as an ornamental and edible water plant.  

What type of environment does it grow in?

Water caltrop prefers temperate climates but can grow in tropical regions.  It grows best in slow-moving, fresh water bodies up to 5 m deep. Though usually it is in water bodies such as dams, ponds and lakes between 30 cm and 3.6 m deep.

Trapa nutans grows best in water with high nutrients, but does not tolerate salinity.

Maps and records

• Recorded presence of Water caltrop 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?

Water caltrop is usually introduced to an area through intentional planting by humans.

By seed

Water caltrop produces heavy seeds that are released in winter and quickly sink. Seeds germinate in water over 12°C, usually within the first 2 years, but may stay dormant for up to 12 years.  The seeds lose their viability if they dry out. 

In Trapa nutans, fresh seeds need cold conditions (4°C) for at least 9 weeks to break dormancy.

A single seed can produce 10 to 15 rosettes and each rosette can then produce up to 15 to 20 seeds. 

The barbed seeds can be spread by clinging to:

• waterbirds and other animals
• fishing nets and traps
• boats
• clothing
• vehicles that go in the water.   

By plant parts

Floating parts of the plant break away from the stem. the plant parts are spread by flowing water and by clinging to boats and equipment. 

References

CABI (2020). Invasive Species Compendium, Data Sheet: Trapa natans (waterchestnut)  https://www.cabi.org/isc/datasheet/55040 

Global Invasive Species Database (2006) Trapa species, http://www.issg.org/database

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

Phartyal, S. S., Rosbakh, S., & Poschlod, P. (2018). Seed germination ecology in Trapa natans L., a widely distributed freshwater macrophyte. Aquatic Botany, 147, 18-23.

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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.

Hand removal, herbicides and mechanical removal have been used to control water caltrop in other countries, but the ability of the seeds to lay dormant for many years makes total eradication very difficult.

Herbicide options

See Using herbicides for more information.

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: One part product to 50 parts water
Comments: Spot spray
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Various products)
Rate: One part product to 9 parts water
Comments: Splatter gun
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate

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Diquat 200 g/L (Reglone®)
Rate: 5 L of product per megalitre of water
Comments: Apply by injection below the surface or as a surface spray.
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: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
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 All species in the Trapa genus are Prohibited Matter in NSW

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Reviewed 2023