Water caltrop (water chestnut)
Trapa natans
Botanical family: Lythraceae
Trapa natans forms a floating rosette of diamond-shaped leaves with petioles swollen into a float, a strange and recognisable beauty. Neolithic archaeological remains in Switzerland, Germany, France (pile-dwelling sites). Harvest of the fruits in September-October, when they sink ripe. A taste between chestnut and potato, sweet and starchy, hence the popular name 'water chestnut'. Excellent keeping after drying on cooking.
Full sun
Deep mud (50-150 cm of water), rich in organic matter
Annual floating plant, permanent water
Annual that self-seeds from the fruits that drop in autumn
Insect-pollinated (small, inconspicuous white flowers)
Role in permaculture
A food plant of deep still waters, forgotten in our gardens: non-invasive in a managed pond, it adds a harvest of starchy seeds, enclosed in a horned fruit with 4 sharp points. On the other hand, never let water primrose or Nuttall's waterweed settle on the edges: these invasive alien species, banned from sale in France, smother the pond.
Good companions
Provide shade or shelter
Traditional companion planting (benefit not demonstrated)
Poor companions
Invasive species: smother the habitat
Insects attracted or useful
Seasons & availability
An annual floating rosette: it flowers in summer and gives its horned, starchy fruits in September and October, which can be dried to keep through the winter.
Available: January, February, September, October, November, December.
Guidance for the oceanic / temperate climate; it may vary with your region, altitude, exposure and growing method.
Sources
- Plants For A Future, « Trapa natans » (pfaf.org/user/Plant.aspx?LatinName=Trapa+natans)
- Gilhodes E., Prud'homme F., Briane G., Cayssiols N. (2019), « La châtaigne d'eau, Trapa natans L. 1753 : questions autour d'une expérience de protection en Aveyron et Midi-Pyrénées », Isatis 19
- FloreAlpes, « Trapa natans / Chataigne d'eau » (florealpes.com/fiche_trapanatans.php)
Nutritional & medicinal properties
For educational purposes. Not a substitute for medical advice. Effects depend on the form, the dose and personal context.
Precautions & contraindications
Never eat it raw: the seed can carry, in water contaminated by human or pig sewage, the metacercariae of the worm Fasciolopsis buski (fasciolopsiasis), complete cooking in boiling water (several minutes) is essential. As the plant grows in water and sediments, its tissues can accumulate heavy metals, and the pond water, even fed with roof rainwater, is not free of PFAS: according to Cousins et al. (2022, Environmental Science & Technology), PFOA levels in rainwater exceed US drinking-water health advisories even in remote regions, and PFOS levels often exceed them. It's therefore better to assess the quality of the water before any regular food harvest (see the concept 'Rainwater quality'). Not to be confused with the Chinese 'water chestnut' (Eleocharis dulcis), a different plant. For any regular food use or health question, refer to professional advice.
Species & varieties
The best-known species and varieties in France.
Species
Voices of peoples
Paraphrases attributed to their documented sources. Never an invented quote.
Neolithic populations of the Alpine pile-dwellings · Switzerland, northern Italy, eastern France, 4th and 3rd millennium BC
On the pile-dwelling sites of Lake Neuchâtel, Lake Constance, Lake Bourget, archaeologists find by the thousand the horned fruits of Trapa natans, charred near the hearths. For five thousand years, the inhabitants of the lake cities boiled these seeds to extract the starch. A massive food dependence, which the Europeans totally forgot in two centuries of agricultural industrialisation.
Hummel M. & Kiviat E. (2004), « Review of world literature on water chestnut with implications for management in North America », Journal of Aquatic Plant Management 42:17-28 ; archéologie palafittique alpine, Convention UNESCO de 2011 sur les sites palafittiques préhistoriques autour des Alpes.
Regulatory status in Europe
A species' status varies from one country to another: check before growing, selling or foraging it outside France.
FranceProtected species
A species protected by decree in several French regions: wild gathering is prohibited there. Obtain seeds or plants through the horticultural trade; growing it yourself remains free.
Arrêtés régionaux de protection de la flore ; statut consolidé par l'INPN (MNHN)
Before you forage
This plant is often gathered from the wild. Only pick what is abundant, never a protected species, and never uproot a whole plant (roots) without the landowner's permission. Rules vary by country and region, check locally.
Key takeaways
- • Prefers full sun exposure.
- • Ideal soil: deep mud (50-150 cm of water), rich in organic matter.
- • Classic companion: Water lily (which shades).
- • Keep away from: Water primrose.