There have been many, varied views amongst botanistis on how to deliminate the various species in the genus Trapa, with interpretations ranging from almost 80 distinct species right down to the concept that there is only one, highly vaiable species. We are following the interpretation in Plants of the World online (see https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:28563-1, accessed 01/05/2024), which accepts 8 distinct species in the genus, including the highly variable Trapa natans which is itself divided into 8 varieties[].

Common Name: Water Chestnut

General Information

Trapa natans is an annual aquatic herb with a rosette of floating leaves and submerged, paired but not opposite, pinnatisect and leaf-like adventitious roots[]. The plant can be free-floating in the water, or rooting in the mud in shallow water. It is also able to grow out of water in very wet, muddy soils[].
Often called water chestnut (though this name properly refers to a quite different plant, Eleocharis dulcis). The plant has a long history of use, with evidence showing that the seeds were a major component of the human diet several thousand years ago, Its use as a food crop has greatly declined in recent times in Europe, but it is a very important crop in many parts of Asia, especially China and India where it is widely cultivated for its nutritious, edible seeds and has various medicinal properties[, ]. The plant is also grown as an ornamental in aquaria and outdoor ponds.
Trapa natans is a widespread and does not face any major global threats. The plant is classified as 'Least Concern' in the IUCN Red List of Threatened Species(2019)[].
Trapa natans is a productive, annual, floating-leaved plant which has been cultivated globally for the nutritious nut it produces. It is an extremely important food crop in China and India and is a protected species in Europe, but in its introduced range (particularly eastern N. America)), it grows in thick stands that displace native vegetation and affect water quality. Thick beds of water chestnut can cause significant declines in dissolved oxygen that negatively affect sensitive fauna. The nearly impenetrable mats are of virtually no use to wildlife and interfere with boating, fishing and swimming, while the large, spiny nuts can cause injuries to swimmers. The plant sets abundant seed, making it difficult to eradicate once it is introduced. The plant is designated as invasive in eastern N. America.[].

Known Hazards

The raw seed contains toxins but that these are destroyed in the cooking process[, ].

Botanical References

, ,

Range

Widespread through temperate and tropical Eurasia and Africa; from western Europe to Japan and Indonesia and from northern to southern Africa

Habitat

Slow-moving rivers, lakes, swamps and ponds; at elevations from near sea level to 2,700 metres[]. Growing in water up to 60cm deep.

Properties

Weed Potential	Yes
Conservation Status	Least Concern
Edibility Rating
Medicinal Rating
Other Uses Rating
Habit	Annual
Height	0.00 m
Pollinators	Insects, Self
Self-fertile	Yes
Cultivation Status	Cultivated, Ornamental, Wild

Cultivation Details

Trapa natans has a very wide native range, being found in the temperate and tropical zones of Europe, Asia and Africa and at elevations up to 1,500 metres. It grows best in areas where annual daytime temperatures are within the range 15 - 25°c, but can tolerate 10 - 35°c[]. When dormant, the plant (in the form of a seed) can survive temperatures down to about -25°c, but young growth can be severely damaged at 0°c[]. It prefers a mean annual rainfall in the range 500 - 1,200mm, but tolerates 300 - 1,600mm[].
A water plant, usually growing in water up to 60cm deep, but recorded to tolerate depths from 30 - 360cm[]. It requires a sunny position in slightly acidic water[]. Dislikes calcium rich water[]. Prefers a rich soil[, , ]. Prefers a pH in the range 5.5 - 7.5, tolerating 5 - 8.2[].
In India, the fruit is broadcast in 30 - 69 cm deep nursery ponds and pressed into the mud, transplanted at the 4 - 5 leaf stage and replanted when 4 - 5 months old. Lateral pruning is carried out to accelerate flowering and fruiting[]. In China, the water chestnut is cultivated in running water[].
The plant is known to fix nitrogen from the water[].
Trapa natans is a very variable species that is recognized as comprising 8 distinct varieties (see notes above on taxonomy). All the varieties will produce edible seeds as per the species, though we only have clear records for some of them. The varieties are:-
Trapa natans var. africana Brenan. Native to eastern tropical Africa, mainly in Lake Victoria.
Trapa natans var. bispinosa (Roxb.) Makino. One of the cultivated varieties, see separate record sheet for more info.
Trapa natans var. japonica Nakai. Found in the temperate climate of Japan and Korea.
Trapa natans var. magnicorona (Z.T.Xiong) B.Y.Ding & X.F.Jin. Found in the temperate climate of Russian Far East, eastern China, Japan and Korea.
Trapa natans var. natans. The base variety upon which this record sheet is based.
Trapa natans var. pseudincisa (Nakai) ined. Found in the temperate climate o Russian Far East, easern and central China, Japan and Korea,
Trapa natans var. quadricaudata (Glück) B.Y.Ding & X.F.Jin. Found mainly in subtropical climates from Russian Far East, Japan, eastern China and southeast Asia, with outliers in Africa.
Trapa natans var. quadrispinosa (Roxb.) Makino. This form is known to be cultivated in China, see separate record sheet for more info.
There are some named varieties[, ].

Edible Uses

Seed - raw, cooked or dried and ground into a powder[, , , , , , , ]. A sweet floury and agreeable flavour[], similar in taste to sweet chestnuts (Castanea spp)[]. The seed contains up to 50% starch according to one report[], 16% starch, 3% protein in another report[] and 15% protein, 7.5% fat in a third[]. (Are all these reports talking about the same thing?). One report says that the raw nut contains toxins that are destroyed by cooking the seed[].

Medicinal

The fruit (seed) is used in Ayurvedic medicine as an antidiarrhoeal, aphrodisiac, appetizer, astringent, diuretic, nutrient, refrigerant and tonic[]. It is also useful in the treatment of lumbago, sore throat, bronchitis, bilious affections, fatigue and inflammation[].
The fruits are used in the preparation of liniments to treat elephantiasis, pestilent fevers, rheumatism, sores, sunburn and skin complaints[, ].

The juice of the stems is used to treat eye disorders[].

The plant is used medicinally to treat rabies, poisonous animal bites, diarrhoea, amoebic dysentery and other complications[]. It has allso been used in a herbal mixture that has proven to provide relief from the symptoms associated with recurrent herpes genitalis and labialis[].

The rind of the fruit has been discovered to have antibacterial activity, and is primarily effective against gram negative bacteria[]. A methanol extract of the fruit rind has shown a hypoglycaemic and antidiabetic activity, reducing and normalizing blood glucose levels[].

Agroforestry Uses:

The plant is known to fix nitrogen and phosphorus from the water[]. It has the potential to be used as a tool to reduce eutrophication, however, the vegetation must be removed annually prior to its decay in order to prevent the subsequent release of these sequestered nutrients back into the water[].
The plant may also be used in environmental reclamation projects, as it is capable of accumulating heavy metals, although not at levels as high as some other species commonly used in this capacity[].

Other Uses

The seeds are sometimes used as beads in necklaces and rosaries[, ].

The plant is a potential source of biomass. It has been recommended for use as paper pulp, fertilizer, compost and biofuel[].

Propagation

Seed - harvest in late summer and store overwinter in a jar of water in a cold but frost-free place. The seed can remain viable for up to 10 years in the soil at the bottom of ponds etc, though it quickly loses its vitality if it is allowed to become dry. Sow in spring, placing one seed in each pot and submerging them under a few centimetres of water.