Trapa natans aka Water Chestnut

Trapa natans is an aquatic herb in the Lythraceae family. Common names include water chestnut, horn nut, and water caltrop. It comes from the Old World, including Europe and Asia. It is not native to Britain or Ireland. This water chestnut differs from Eleocharis dulcis, which shares its common name. Useful Tropical Plants describes it as annual; PFAF calls it perennial. PFAF advises growing it as an annual in Britain.

The plant forms a rosette of leaves on the water. It can float freely or root in shallow mud. It also grows in very wet mud outside standing water. Submerged adventitious roots look like leaves and are pinnatisect, or deeply divided. These roots grow in pairs, but not opposite each other. Small white flowers appear in June and July. Flowers have both male and female parts, and plants are self-fertile. Insects also pollinate them. The fruit has a hard shell around an edible seed.

It needs full sun and cannot grow in shade. PFAF gives a water depth of up to 2 feet [0.6 metres]. It prefers rich soil and slightly acid water. Calcium-rich water does not suit it. It accepts sandy, loamy, and clay soils with acid or neutral pH. PFAF lists USDA zones 6–12 and reports broad hardiness in Britain. Growth is fast, and plants can cover large areas. Gardeners grow it in ponds and aquaria for its floating foliage.

People have eaten its seeds for thousands of years. It remains a food crop in China and India. The seeds have a sweet, starchy taste compared with chestnuts. They can be cooked, dried, or ground into flour. PFAF warns that raw seeds contain toxins which cooking destroys. Its fruits also have a history of use in medicinal liniments. These traditional uses do not establish their effectiveness. People sometimes use the seeds as beads for necklaces and rosaries.

Its spread can become a serious problem outside its native range. PFAF reports that it can crowd out native plants. It can also form stands dominated by this one species. Useful Tropical Plants reports a global conservation status of Least Concern. For propagation, collect seeds in late summer and keep them wet. Drying soon destroys their ability to germinate. Store them over winter in cold, frost-free water. Sow them in spring under a shallow layer of water.