- Type
- Grains
- Difficulty
- Hard
- Season
- Summer·Fall
- Sowing
- From seed
Water Caltrop
Trapa japonica
Starch and polyphenols in the fruit of a water plant, not a cereal grain
Water caltrop is an annual aquatic plant that floats its leaves on the surface of ponds and rice paddies. Under the water a hard, horned fruit ripens, and the white kernel inside is boiled and eaten. The skin of fruit grown in water can carry the encysted larvae of flukes, which is why it is never eaten raw: boil it thoroughly in the shell, and cut the hard shell open with a knife rather than cracking it with your teeth. The Donguibogam records it as 菱仁, calling it neutral in its own entry while the herbal it quotes says nothing grown in water is colder and warns against eating much of it. The same passage notes that boiling and grinding the kernel gives a flour that is strikingly white and smooth.
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Where It Grows
- Main producer Known as lingjiao, it is widely grown at the water's edge in Jiangsu, Zhejiang and Anhui.
- Main producer Known as singhara, it is grown in the ponds of Uttar Pradesh and Bihar. The plant here is Trapa natans var. bispinosa, a different form.
- Wild population It grows wild on the surface of ponds, reservoirs and paddy fields throughout the country.
- Wild population It grows in ponds and marshes from Honshu southward.
It occurs widely in the standing water of temperate and subtropical Eurasia, and in East and South Asia it is grown as a crop for its edible fruit. The genus Trapa splits into several forms according to the number of horns and the shape of the fruit, and opinion is divided over whether to treat those forms as species or as varieties; grouping Korea's water caltrop (Trapa japonica) with the plants grown in China and India under one name blurs the map. It has also spread to Europe and North America, but there it is treated as an invasive species that blocks waterways, so those regions are not counted among the growing areas here.
- Optimum
- 20~30℃
- Range
- 15~35℃
- Hardiness
- Not hardy
An annual aquatic plant that spreads its leaves out across the water surface in summer. It sprouts from the bottom in spring as the water warms, grows through the summer, drops its fruit in autumn and then rots away. The fallen fruit overwinters under the water, so if the water dries up there is no following year.
In Korea Nationwide
It grows wild on the surface of ponds, reservoirs and paddy fields. The seed is placed on the bottom of the water in April–May, and the ripened fruit is lifted out in September–October.
How to grow
- Light
- Six or more hours of direct sun.
- Soil and depth
- Lay 10 cm of paddy soil or loess in the bottom of the tub, then fill with water.
- Spacing
- One or two plants per 50 cm tub.
- Watering
- Keep the water at least 30 cm deep.
- Feeding
- None.
- Harvest
- Feel below the leaves by hand in September and October.
What makes the difference. The seed dies if it dries out. Fruit kept for next season has to overwinter submerged in water to germinate. When cooking, always boil it in the shell for 20 minutes or more.
Why this one is harder. It cannot be grown in a soil pot. You need a basin or tub holding at least 30 cm of water.
Common problems
- Aphids You will see Black insects cluster on emergent leaf stalks and new leaves What to do Rinse them off with a stream of water. Skip sprays in a closed tub
- Pond snails You will see Holes appear where leaf undersides have been grazed What to do Pick them out on sight. Dry the soil in the sun before use to kill the eggs
This page describes how a crop is grown and used. It does not claim health effects for any food, nor is it intended to prevent or treat disease. Component and study notes are references only and do not mean the same result occurs in people. Consult a medical professional about health concerns.
Health Benefits
What the old text says. Water caltrop appears in the Donguibogam's fruit section as neungin (菱仁), recorded as neutral in nature, sweet in taste, and free of toxicity. The herbal quoted in the same passage adds that nothing grown in water is colder and that eating much of it swells the belly. This is a record from a seventeenth-century medical text, not a modern medical judgement.
Nutrition
- Carbohydrate (starch) (About 20 g per 100 g boiled) — An energy source closer to grain than to fruit
- Potassium (Comparatively high) — A mineral common in plant foods
A starch crop, not a vegetable The kernel sits closer to a grain than to a vegetable. Around 20 g of carbohydrate per 100 g boiled means it eats more like part of a meal than a snack. Fat stays under 1 g, so it behaves nothing like a nut despite the look.
Potassium and minerals Potassium runs comparatively high. Potassium is the mineral that helps the body clear sodium. Magnesium and phosphorus come along with it.
Pairings
○ Glutinous rice and red beans — Mash boiled kernels into a porridge with glutinous rice or red beans and the starches soften into each other. Water caltrop on its own tends to stick in the throat, and grains solve that. Adzuki Bean
○ Ginger — The Donguibogam wrote that if eating too much water caltrop leaves the belly swollen, ginger-steeped wine settles it at once. It is the old approach of countering something cold with a warming aromatic. Ginger
△ Eating it raw — Never eat water caltrop raw from a wild pond. Fruit that grows underwater can carry the encysted larvae of flukes on its skin, and eating unheated aquatic plants is a known route of infection. Boil it in the shell for 20 minutes or more, without exception. Cut the shell with a knife instead of cracking it with your teeth, and wash your hands and the board straight after. Treat other water-grown crops such as water chestnut and lotus root the same way.
△ Large amounts at once — The Donguibogam called it the coldest thing grown in water and said too much of it swells the belly. Between the starch and the tannins it genuinely is slow to digest.
Varieties
- Water caltrop (Seed) — The native species. The fruit carries four horns.
- Dwarf water caltrop (Seed) — A small type with smaller leaves and fruit. Fits a water bowl.
Source: Donguibogam, Fruit Section (菱仁) · National Institute of Biological Resources · Rural Development Administration (nongsaro.go.kr)