Gathering water caltrops in autumn is the liveliest waterborne harvest: a wooden basin rows into the caltrop pond, turns over the rosettes of leaves one by one, and under each hangs a little horned treasure boat. Crack the hard shell and the flesh is snowy white, crisp and sweet.

The Autumn of Caltrop Gathering

Gathering water caltrops in autumn is the liveliest waterborne harvest: a wooden basin rows into the caltrop pond, with the picker half-crouched inside, turning over the leaf rosettes two-handed, each one revealing little horned treasure boats underneath. Crack the hard shell and the flesh is snowy white, crisp and sweet, the cool creeping up from the fingertips.

You must gather caltrops early and judge between tender and ripe: green-shelled, tender ones are best eaten raw; firm ones are set aside for boiling. Turning over one pond’s rosettes, a layer piles up in the basin; by the time the sun climbs, your back is soaked through.

— Bai Juyi, Tang Dynasty, “Song of Picking Lotus” —

Caltrop leaves ripple the waves, lotus sway in the wind,

deep among the lotus blossoms a small boat passes.

The Old and the Tender

Old and tender caltrops are two different ways to eat: tender ones are cracked open and eaten at once—crisp and juicy; ripe ones are washed, boiled in a pot, powdery and fragrant, cracked open as a snack that keeps you seated for an afternoon. Still older ones are ground into powder—the caltrop starch of old times—stirred into a smooth, faintly sweet bowl.

In Jiangnan, caltrop was the taste of old autumn days. In the gathering season, boat songs rise over the water and make their way into poetry as caltrop-gathering songs—one pond of water, one boat of people, all singing about the season’s harvest.

How It Floats

The middle of the caltrop’s leaf stalk bulges into an air bladder, like a little float tucked inside, holding the rosette of diamond-shaped, toothed leaves steadily on the surface while the roots anchor in the mud below. The leaves arrange in rings of rosettes, spreading flatter toward the edges.

The horned fruit is its sinking device: after ripening, the caltrop sinks to the pond bottom to winter over, sprouts in the mud next year, and slowly rises to the surface—a single caltrop’s life is one round trip, down and up.

• How to know it: Annual floating-leaved herb; leaves floating, diamond-shaped and toothed; flowers small and white; fruit nutlike with horns, commonly called the water caltrop.
• Season: Flowers in summer; fruit ripens September to October; caltrop gathering is the most famous waterborne autumn harvest of Jiangnan.
• Caution: If eaten raw, wash and peel carefully; caltrops from wild water bodies are best not eaten raw; those with weak spleen and stomach should not overeat.

Uses on the Water

The fruit is eaten and made into starch; the stems and leaves serve as silage feed. A caltrop pond is itself a layer of aquatic vegetation that shades, cools, and dampens wind and waves, benefiting the pond’s mud and water alike.

But a caltrop pond needs management: if it grows too dense it smothers the surface and cuts dissolved oxygen. The old practice was annual pond turnover, section by section, leaving breathing gaps—the harvest on the water also depends on measure under the water.

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Keywords: water caltrop · caltrop · water-town autumn harvest · floating-leaved plant