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Water caltrop (Trapa natans)

Water caltrop is not known to occur in Australia, but all species in the Trapa genus are prohibited matter in NSW and must be reported if found. It can form dense mats that block waterways and harm native plants, aquatic animals and water access.

Key facts

Type: Weed, aquatic.

Native to: Warm temperate parts of Eurasia and Africa.

Established in: .

Other names: floating water chestnut, bat nut.

Identification

  • It is an annual water weed that dies over winter and grows again from seed in spring.
  • Floating leaves form glossy rosettes and have toothed edges, fine hairs underneath and stalks that usually have a swollen bulge.
  • Its submerged leaves are feather-like and arranged in whorls around the stems; its white, four-petalled flowers appear above the water in early summer.
  • The plant produces hard, brown or black nuts about 3 cm wide, with sharp spines and one seed; T. natans var. natans has four spines and T. natans var. bispinosa has two.
  • Its submerged, unbranched stems can reach 3.6–4.5 m long and are anchored in mud by fine roots.
  • Look-alike: Mosaic flower (Ludwigia sedioides) can look similar, but it has yellow flowers about 3 cm wide and does not have inflated leaf stalks.

Where it grows

Water caltrop is not known to occur in Australia, although archival evidence records it being grown in Brisbane by the Queensland Acclimatisation Society in the late 1800s.

Why it matters

  • It can form thick, impenetrable mats across wide areas of water and block access.
  • It outcompetes native plants and reduces food and habitat for fish and other aquatic animals.
  • Sharp spines on the nuts can injure people and animals.
  • Dense mats can prevent activities such as swimming and fishing.

How it spreads

  • It is usually introduced to an area through intentional planting by people.
  • Heavy seeds sink in winter; they can remain dormant for up to 12 years, and each seed can produce 10 to 15 rosettes.
  • Barbed seeds can cling to waterbirds and other animals, fishing nets and traps, boats, clothing and vehicles that go in the water; floating plant parts can also spread in flowing water or cling to boats and equipment.

Control options

  • Do not attempt to treat or dispose of this weed yourself.
  • If you see it anywhere in NSW, report it immediately to the NSW Biosecurity Helpline on 1800 680 244.
  • NSW DPIRD, Agriculture and Biosecurity will lead the initial response for treatment and disposal.

Hygiene and washdown

  • Barbed seeds can cling to boats, fishing nets and traps, clothing, and vehicles that enter the water.
  • Floating plant parts can cling to boats and equipment.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Prohibited matter. Report it to Biosecurity Queensland on 13 25 23 within 24 hours and do not move or deal with it. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Prohibited matter. It is an offence to deal with it, and you must notify NSW DPIRD as soon as you become aware of it.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • Western Australia: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • Tasmania: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • Northern Territory: Check NT list. There is no NT weed profile for it. Check the NT declared weeds list for its current status.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

Sources