Trapa natans L.

Growth Habit and Habitat

Trapa natans L. is an annual aquatic plant that grows in shallow, slow-flowing fresh water, rooted in bottom mud.

Submerged Stem and Leaves

It produces a submerged stem up to approximately 2 meters (7 feet) long and 6 mm (0.24 inches) thick that bears thin submerged leaves.

Floating Leaf Rosettes

The plant forms floating leaf rosettes that reach around 50 cm (20 inches) across.

Floating Leaf Morphology

These floating surface leaves are diamond-shaped with toothed edges, growing up to 5 cm (2 inches) long and 6 cm (2.5 inches) wide, and attach to petioles (leaf stalks) that can be up to 21 cm (8.3 inches) long.

Flower Structure

Its flowers have 4 white to pink petals up to 10 mm (0.4 inches) long, 4 sepals, 4 stamens, and one pistil.

Ovule Development

The ovary starts with 2 ovules, but only one matures.

Pollination

Flowers are mostly self-fertilizing, and are occasionally pollinated by insects.

Fruit Structure

The fruit is a single-seeded drupe classified as a nut, which develops 2 to 4 barbed spines 12 mm (0.5 inches) long from the original sepals.

Chemical Composition

This plant contains an ellagitannin polyphenol called bicornin.

Preferred Habitat Conditions

This species prefers warm, sunny, nutrient-rich areas with soft, organic-rich bottom mud.

Temperature Requirements and Range Shift

It requires water temperatures above 22 °C (72 °F) for at least 2 months each year, and is currently expanding its range northward due to climate change.

Edibility of Fruits and Seeds

The fruits and seeds of Trapa natans are edible raw or cooked.

Prehistoric European Use

Archaeological discoveries from southern Germany suggest prehistoric populations in the area may have relied heavily on wild water caltrops to supplement their regular diet, and may have used these plants as their main food source when cultivated cereal crops failed.

Historical Chinese Use

It has been used as an important food for prayer offerings in China since the Zhou dynasty; the second century BC text The Rites of Zhou instructed worshippers to carry bamboo baskets filled with dried water caltrops for offerings.

Cultivation in South Asia

In India and Pakistan, it is called singhara (or paniphal in eastern India), and is widely cultivated in freshwater lakes.

South Asian Culinary and Ritual Use

Its fruits are eaten raw or boiled; dried fruits can be ground into a flour called singhare ka atta, which is used in many religious rituals, and can be eaten as part of the phalahar (fruit diet) observed during the Hindu fasting period of navratas.

Historical European Market Presence

Water caltrops were sold in markets across Europe until 1880.

Italian Historical Preparation

In northern Italy, the nuts were sold roasted similarly to how sweet chestnuts (Castanea sativa) are sold today.

European Population Decline

Water caltrops were widely used as human food across many parts of Europe until the early 20th century, but the species has since become rare and nearly extirpated in Europe.

Causes of European Decline

This decline has been linked to several factors: climate fluctuations, changes to the nutrient content of water bodies, and the drainage of many wetlands, ponds, and oxbow lakes.