Water caltrop

Photo: Okapi — CC BY-SA 3.0 · source

Survives zone 5 through 9

Harvest calendar

Ripens: Mid-July to Oct, once water stays above 20°C

Best planted in early spring after your last frost, or in fall before the ground freezes, depending on your climate.

Grow this one only where it is legal to. In the United States it is a federal noxious weed — sale, purchase and interstate transport are illegal under federal law, and most states go further: Massachusetts bans possession, transport and cultivation outright, New York, Vermont and Connecticut require a permit even to grow it, and roughly 35 states list it as noxious. The seed itself is what people eat, floury and sweet-chestnut-like once cooked, but it also concentrates heavy metals from the water it grows in, so the water source matters. In the UK and continental Europe it is a long-cultivated pond plant with no comparable restriction. If you are in the US, check your state's noxious-weed list before sourcing seed — this entry documents the plant, not a recommendation to plant it there. ⚠ Always cook it. Raw or undercooked water caltrop is a documented route for the intestinal fluke Fasciolopsis buski, whose cysts sit on the surface of the nut and are destroyed by cooking.

Spread warning. US federal noxious weed; illegal to sell, buy or transport across state lines. Forms dense floating mats that block waterways, starve the water of oxygen and crowd out native aquatic plants — one plant can produce over 100 rosettes and 10-15 seeds a week at its peak. Prohibited or restricted in most US states; not restricted in the UK or most of continental Europe, where it is native.

Hardiness

Zone 5–9

Habit

Herb

Edible part

Seed

Ripens

Mid-July to Oct, once water stays above 20°C

Mature size

Floating rosette 15–50 cm across; a single plant can spread to over 100 rosettes in a season

Light

Full sun

Soil pH

6.0–7.5 (still or slow-moving fresh water, up to about 60 cm deep)

Environment

Temperate freshwater ponds and slow rivers

Native range

Eurasia and Africa — still and slow-moving freshwater ponds, lakes and river margins; introduced and naturalized in parts of North America.

Status

Both wild and cultivated

Years to fruit

1 yr

Growth speed

Very fast — one rosette becomes a hundred in a season, which is the invasiveness problem in one sentence

Spacing

100 cm (39 in) apart — about 1 m² (11 sq ft) per plant

Leaf shape

Diamond-shaped, toothed, floating in a surface rosette, each leaf's petiole swollen with air chambers for buoyancy

Root system

Two kinds at once: fine feeding roots anchoring in the mud below, and feathery green photosynthetic roots along the submerged stem. Plants often break free and float on, rooting again elsewhere — part of why it spreads so aggressively.

Fruit description

A hard, four-horned (in this species; two-horned in T. bicornis) black nut containing one edible seed, floury and sweet once cooked

Propagation

Seed, harvested in late summer and kept submerged in water through winter (it dies quickly if allowed to dry out), then sown in spring just under the water surface.

Pollination

Self-fertile — flowers routinely self-pollinate before they even open, though insects visit them too.

The edible parts listed above are a starting point, not a field guide. Identify a plant with certainty — ideally against a local authority — before eating any part of it, and read the full disclaimer. Spotted something wrong? Report it.