Water caltrop (Trapa species)

PROHIBITED MATTER: If you see this plant report it to the NSW Invasive Plants & Animals Enquiry Line 1800 680 244

Water caltrop is a floating aquatic plant that grows in slow-moving water up to 5 m deep, with its stems rooted in the soil beneath the water.

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How does this weed affect you?

Water caltrop is a floating aquatic plant that grows in slow-moving water up to 5 m deep, with its stems rooted in the soil beneath the water.

Water caltrop forms large populations that can create nearly impenetrable mats across wide areas of water, out-competing native plants and making waterways inaccessible. Sharp spines on the fruit create a hazard for humans and animals.

Water caltrop has been cultivated around the world as an ornamental water plant, and as such, it is likely to be found in farm dams, water features and fish ponds, or in ponded and slow-moving water bodies near towns.

Two species of the Trapa genus—Trapa natans and Trapa bicornis—are referred to as water caltrop. Water caltrop is also known as water chestnut, but is not related to Chinese water chestnut (Eleocharis dulcis), which is cultivated for its edible tubers.

Where is it found?

Water caltrop is native to the warm temperate parts of Eurasia and Africa.

In China the seeds are cooked and eaten. Various Trapa species have been introduced into North America and have become invasive in eastern areas of Canada and the United States.

Although water caltrop is yet to be found in New South Wales (NSW) it has the potential to become a significant weed.

How does it spread?

Water caltrop drops seeds during winter. The seeds germinate in the mud during the warmer months, and grow stems that reach the water surface and produce rosettes. A single seed may give rise to 10 to 15 rosettes. Each rosette can produce up to 15 to 20 seeds.

The plant spreads when rosettes break allowing the fruits to detach from the stem and float away. The fruit can also be spread by birds and other animals. Seeds can remain viable for up to 12 years, although most will germinate in the first two years. If the fruit dry out they will not contain viable seeds.

What does it look like?

Trapa bicornis is similar to Trapa natans, but has two spines on the fruit instead of four.

Key identification features

  • Stems are submerged, unbranched and long, reaching 3.6–4.5 m in length. The stems are anchored into the mud by very fine roots.
  • There are two types of leaves. The first are submerged and fall off during early stem growth. They are finely divided and feather-like. The second form on the water surface in a rosette. These have saw-toothed edges and are oval- or triangular- shaped, 2–3 cm long, with glossy upper surfaces and lower surfaces covered with fine short hairs.
  • Feather-like roots up to 8 cm long develop where the submerged leaves drop off, and are often mistaken for feather-like leaves.
  • White, four-petalled flowers are 8 mm long, and form above the water surface in early summer.
  • The fruit is a woody or bony nut, about 3 cm wide, with two or four (1 cm long) stout spines or horns. Each fruit contains a single seed.

Acknowledgements

Produced by: Annie Johnson, Andrew Petroeschevsky, Stephen Johnson, Elissa van Oosterhout, John Hosking, Rod Ensbey, Birgitte Verbeek

References

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

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

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Control

Contact your local council weeds officer for assistance if you suspect you have found water caltrop. 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

Contact your local council weeds officer for control advice for Water caltrop (Trapa species).

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

Reviewed 2017