Water Caltrop

Trapa is a floating annual aquatic plant with rosettes of diamond-shaped leaves on the water surface and submerged, feathery stems. After the small white flowers, it develops hard, woody fruits armed with two or four sharp spines, shaped like a bull's head or bat in some species. Each fruit contains a single large seed with firm, white, starchy flesh. The kernel has a mild, slightly sweet taste, similar to chestnut or sweet potato when cooked. The fruits sink to the bottom when mature, where they overwinter before germinating the following spring.

Water caltrops grow in slow-moving freshwater lakes, ponds and rice paddies in Europe, Asia and Africa. Trapa natans is native to temperate Eurasia, while Trapa bicornis, the ling jiao, is cultivated in China and Southeast Asia, and Trapa bispinosa is cultivated in India and Kashmir. The plant has been introduced to North America, where Trapa natans has become an invasive species in northeastern lakes and rivers. Today the nut is cultivated in China, India, Thailand and Taiwan, and wild collection continues in some regions.

Archaeological finds show that water caltrop was eaten by people in Neolithic Europe and Asia, with remains found in lake dwellings in Switzerland, Germany and Italy, and in ancient Chinese sites. Ancient Greek and Roman writers refer to it, and in medieval Europe, the nut was eaten roasted and ground into flour and was sometimes called the water chestnut.

In India, singhara flour made from the dried kernel is a major food for religious fasting periods such as Navratri, when grains are avoided, and is used for pancakes, puris and sweets. In China, fresh and boiled caltrops are sold in autumn markets, and the plant is a traditional crop in rice-paddy ponds in southern China. In Europe the species has become rare in some countries and is protected, while in North America it is controlled as an invasive plant.

Fresh caltrop kernels are boiled or steamed and eaten as a snack, or roasted over a fire. In India, the dried kernels are ground into singhara atta, used for flatbreads, fritters and halwa. In China and Southeast Asia, they are used in soups, stir-fries and desserts. The starch is used as a thickener, and the leaves and stems of some species are used as animal feed.

Water caltrop kernels are mostly starch and water, with small amounts of protein and very little fat, so they are modest in calories. They provide potassium and some B vitamins. Values vary between species and between fresh and dried kernels. The flour is gluten-free and is used in fasting diets. Traditional systems of medicine in India and China use the plant in various ways, but reliable clinical evidence is lacking. It is considered a regional food rather than a significant nutritional source.