Trapa spp. L.
For information on the currently accepted species of Trapa, see "Notes".
Trapaceae
The genus Trapa has in the past been placed in families Lythraceae and Onagraceae. Family Trapaceae seems to be the currently accepted family, although some authorities still use family Lythraceae.
floating water chestnut, water chestnut, water caltrop.
Note: three species, Trapa natans, "Trapa bicornis" and Trapa rossica are known by the above common names. Trapa rossica is now an endangered species and "Trapa bicornis" is now considered to be a synonym of Trapa natans. For further information, see the topic "Notes".
The genus Trapa is native to warm temperate Eurasia and Africa. In Europe, water chestnuts (Trapa spp.) were common until the beginning of the 20th century, however populations of Trapa spp. are now so rare (or nearly extinct) in much of Europe that in Germany, for example, species of Trapa are listed as endangered.
The genus Trapa is absent from Australia.
Species of Trapa, especially those plants that were separated under the species names of T. natans and T. bicornis (= T. natans var. bispinosa), have been extensively cultivated as an important human food resource. Until the beginning of the twentieth century, floating water chestnuts (four-spined variety) were offered for sale as food in Europe. Research centred on India and China suggests that the two spined variety of T. natans has been cultivated as a food crop in those countries for at least 3000 years. In the United States of America, the original intention was to cultivate T. natans var. natans as a food crop, however it escaped from cultivation and has become an invasive weed species.
Note: according to the Invasive Species Database, the name "Trapa bicornis" is now a synonym of Trapa natans var. bispinosa. See "Notes" for further information.
The species Trapa natans is present as an invasive species in various eastern states of the United States of America. No species of Trapa is naturalised in Australia.
Floating water chestnuts (Trapa spp., water caltrops) grow in stationary or slow moving bodies of fresh water up to 5 m deep.
Floating water chestnuts (Trapa spp., water caltrop) are aquatic plants with triangular shaped, floating, waxy leaves held in a rosette; the surface leaf rosette is borne on a stem that extends to and is rooted in the mud floor of the water body; the stem may be up to 5 m in length depending upon water depth.
Species of Trapa produce stems up to 6 mm in diameter and are commonly about 1 m long, however the length can be up to 5 m depending on water depth. The stem is anchored to the bottom mud by very fine roots. During its growth from the bottom of the water body, the stem produces submerged leaves which are soon discarded and the stem sites from which these leaves have fallen then produce feather like roots which can be easily mistaken for leaves. Up to 10 (occasionally 15) stems can be produced by a single seed and each stem will produce a rosette of leaves on the water surface.
Floating water chestnut plants produce two types of leaves. While the stem is growing to the surface, it produces feather-like leaves which are discarded once the plant reaches the surface. At the surface, the plant produces a rosette of leaves which are waxy, oval/rhomboidal or triangular, glossy on the upper surfaces, coated with short, fine hairs on the lower surfaces, serrated on the margins and normally 2-3 cm long but they can reach lengths of up to 6 cm. The leaf stalks (petioles) are enlarged and spongy with air sacks which allow the rosette to float.
The white flowers are carried on short stalks, have four petals, are about 8 mm long, and are produced at the centre of the rosettes. The flowers are supported by bracts (sepals) and these eventually become the spines on the fruit. The flowers may self-fertilise before opening or they can be insect pollinated. Once fertilisation has occurred, the stalk droops downwards to place the flower into the water where development of the fruit then continues.
The fruit is a hard seed pod or nut (about 3 cm wide) with two or four barbed spines which are about 1 cm long. Each seed pod contains a single seed and one rosette can produce up to 20 seeds in a season.
Reproduction in floating water chestnuts (Trapa spp.) is either by seeds or vegetatively.
Seeds can be carried by water currents or animals and they can remain dormant in the mud for up to 10 years; the seeds do not survive desication. Trapa spp. also reproduce vegetatively: portions of the parent plant can break off and water currents then move them away from the original location - they then survive long enough to produce seeds. Observations indicate that as much as a 10-fold increase in the biomass of a floating water chestnut population can be achieved in a single season and each of the new plants will produce up to 20 seeds.
The heavy seeds sink to the bottom of the water body and do not travel far, however plant fragments can be carried long distances to establish new populations.
Human activity has also contributed to the wide dispersal of these plants whether through the desire for a food crop or ornamental species in dams, lagoons, ponds and aquaria.
Floating water chestnuts (Trapa spp.) can have extremely detrimental effects on a fresh water environment. Severe infestations can produce up to three successive layers of the plants on the water surface and the subsequent reduction of light to the bottom of the water body will eliminate native plants that normally would be present. Oxygen levels in the water beneath the infestation become lower and eventually lethal to fish and other native organisms. The overall effect is the smothering and destruction of the native aquatic flora and fauna. The plant does not provide food for bird life which consequently abandon the infestation area.
The heavy infestations of Trapa spp. can affect human activities such as fishing, boating or swimming. The plants can clog irrigation facilities. In addition, the extremely sharp seeds are capable of penetrating footwear and causing personal harm. The massed plants can also provide better habitat conditions for mosquito breeding.
Currently, almost all of the Australian states place declarations on both Trapa natans and Trapa spp. separately, however in each case, the same declaration status applies to both.
New South Wales. Trapa natans and Trapa spp. are declared Class 1 State Prohibited Weeds. Any infestation must be eradicated and the area kept free of the plant. All outbreaks must be reported to the local council within 24 hours. The plants are prohibited from sale in the state.
Tasmania. Floating water chestnut is a Declared Weed in Tasmania. The importation, sale and distribution of the plants is prohibited in Tasmania.
Queensland. Floating water chestnut is a Declared Class 1 species. The introduction, keeping, and supplying of the plants is prohibited without a permit from Biosecurity Queensland.
South Australia. Floating water chestnut (Trapa natans only) is a Declared Category 1 Weed species. The entry, movement or sale of the plants is prohibited. Land owners are required to destroy any infestation that occurs on their properties.
Western Australia. All species of Trapa are prohibited from entry into the state.
No species of Trapa is known to be present in Australia as at the beginning of 2015. If water caltrop (especially the species T. natans) gains entry, it has the potential to spread across the coastal and adjacent ranges and inland areas from the Kimberley region in Western Australia, across the northern tropics, along the eastern and southern coastlines to South Australia, the southwest portion of Western Australia and finally Tasmania.
Prevention of entry is the best current option, but equally important will be a rapid response to any discovered infestation. The species can be eradicated if sufficient resources are available and such a situation was reported from the state of Virginia in the United States of America.
Large infestations will require both mechanical "harvesting" and herbicidal sprays. In addition, once a mature infestation occurs, annual follow-up procedures over a period of more than 10 years will be required to ensure that all deposited seeds have either germinated or died.
Species of Trapa are not likely to be confused with other water plants. The distinctive rosettes and the spined nuts make the plants easy to distinguish.
The world herbarium database "The Plant List", currently lists 89 species names for genus Trapa, but considers that only 6 of these are accepted distinct species names: Trapa europaea Fler., Trapa flerovii Dobrocz., Trapa incisa Siebold & Zucc., Trapa japonica Flerow, Trapa natans L. and Trapa rossica V.N. Vassil. The species "Trapa bicornis" is not included in this species list because The Plant List herbarium database authority considers the name T. bicornis is synonymous with T. natans.
According to the Invasive Species Database, the name problem is resolved as follows. Trapa natans has two varieties: T. natans var. natans which has four spines on its seed pods, and T. natans var. bispinosa which has only two spines. Trapa natans var. bispinosa then includes the following species as synonyms: T. bicornis, T. bicornuta and T. japonica. Finally, T. natans var. bispinosa is the species that is cultivated for food in most of Asia and India; T. natans var. natans was the species that was used for food in Europe and is invasive in parts of North America.
Current information strongly suggests that of the accepted species of Trapa, only T. natans and its varieties (including the variety known as the separate species of T. bicornis) are recognised as invasive weed species but in any case, most descriptions for Trapa spp. will probably be based on T. natans.
"Floating water chestnuts" (Trapa spp.) should not be confused with another edible plant which is also called "water chestnuts". This second plant is a sedge, Eleocharis dulcis, and it is the crisp corms of this plant that are commonly used in Western-style Chinese food.