Water caltrop (Trapa species)

PROHIBITED MATTER: If you see this plant report it. Call the NSW DPI Biosecurity Helpline 1800 680 244

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.

Profile

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
  • on stalks that usually have a swollen round bulge
  • oval, triangle or diamond-shaped with saw-toothed edges
  • covered in fine, short hairs underneath.

Flowers:

  • are white with four petals
  • are 8 mm long 
  • grow above the water surface 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 a lot like mosaic flower (Ludwigia sedioides), which is sometimes grown in ponds and water features. 

  • Water caltrop has white flowers 8 mm wide and usually has inflated leaf stalks
  • Mosaic flower 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, with the nuts usually boiled or roasted. 

What type of environment does it grow in?

Water caltrop grows in slow-moving, fresh water up to 5 m deep. It grows in farm dams, water features, fish ponds, or in other ponded and slow-moving water bodies. 

Water caltrop grows best in cooler, temperate conditions but will tolerate tropical climates.

Maps and records

  • Recorded presence of Water caltrop during property inspections (Map: Biosecurity Information System - Weeds, 2017-2020)
    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.

  • Estimated distribution of Water caltrop in NSW (Map: NSW Noxious Weed Local Control Authorities, 2010)
    Map shows weed distribution and density estimated by local council weeds officers in 2010.

How does it spread?

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

By seed

Water, birds and animals spread the seed. Nuts can also hook onto equipment like nets, fishing traps and vehicles that go near the water. If the nuts dry out they will not contain viable seeds.

Seeds drop during winter and can remain viable for up to 12 years. Most seeds will germinate in the first two years. Seeds germinate in the mud during the warmer months. A single seed can produce 10 to 15 rosettes and each rosette can produce up to 15 to 20 seeds. 

By plant parts

Floating parts of the plant break away from the stem and float away, or cling to boats and equipment. 

back to top

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

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


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


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: 1 day in pasture, 10 days in treated water.
Herbicide group: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
Resistance risk: Moderate


back to top

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.
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

back to top


For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW DPI Biosecurity Helpline on 1800 680 244 or send an email to weeds@dpi.nsw.gov.au

Reviewed 2020