Water caltrop.

At a glance

Care level

Moderate

Plant type

Aquatic annual

Light

Full sun

Indoor light

Direct sun

Water

Every 0 days

Mature size

2 to 3 feet wide

Growth rate

Fast

Hardiness

USDA 6 to 10

Bloom time

Mid to late summer

Care

Keep this plant in a sunny, calm body of water with plenty of nutrient rich mud at the base. Watch for rapid spread, as it can quickly overtake small water features if left unchecked. Ensure the water remains still, as moving water can hinder its growth.

Watering

This is an aquatic plant that must remain submerged in water at all times.

Feeding

Use aquatic plant fertilizer tablets pressed into the mud near the roots during the growing season.

Pruning

Remove excess rosettes regularly to prevent the plant from covering the entire surface of your pond.

Repotting

Fresh seeds or young plants should be placed in new mud at the start of each spring season.

Soil

Rich organic mud

Soil pH

6.0 to 8.0

Temperature

10 to 30°C

Humidity

70 to 100%

Safety

People

Raw seeds contain tannins and potential toxins that require careful preparation.

Children

Keep children away from the water where this grows, as the sharp, spiny fruits are a significant puncture hazard.

Contact allergens

The sharp spines on the fruit can cause painful physical injury and skin irritation.

Airborne allergens

There are no notable airborne allergens associated with this plant.

Edibility

The seeds must be thoroughly cooked before consumption to neutralize potential toxins.

How to identify it

• Floating rosettes of diamond shaped leaves
• Serrated leaf margins
• Inflated spongy leaf stalks
• Four pronged woody nut fruits
• Small white four petaled flowers

Height

0m

Spread

0.6 to 1.5m

Leaves

green, bronze, deciduous

Flowers

white, 1 to 2cm

Fruit

black, brown, Autumn

Common problems

• Invasive spread — The plant can rapidly choke out native aquatic vegetation and block waterways.

Growing it

Propagation

seed

When to plant

Spring

This plant is highly competitive and usually dominates its space, so it is best kept in isolated containers.

Grows well with

• Water Lily — They share similar aquatic habitat requirements.

Similar species

• Water Chestnut — Often confused due to the common name, though they are entirely different plants.

Science

Family

Lythraceae

Genus

Trapa

Order

Myrtales

Class

Magnoliopsida

Native range

Eurasia and Africa

Conservation

It is considered an invasive species in many regions outside its native range and should not be introduced to new water bodies.

It thrives in stagnant or slow moving freshwater, forming dense mats that shade out other aquatic life.

Value to wildlife

provides cover for aquatic life

Pollinators

bees, beetles

Uses

ornamental pond plant, culinary starch source

Taxonomy resolved against GBIF.

Questions people ask

Is it safe to grow in a garden pond?

It is generally discouraged due to its aggressive growth and the danger posed by its sharp, spiny fruits.

How do I control it?

Physical removal of the plants before they set seed is the most effective method.

Where the name comes from

The genus name Trapa is derived from the Latin word for a caltrop, a spiked military weapon that matches the fruit shape. The species name natans means floating.

Good to know

• The seeds are often called water caltrops or devil pods.
• It is an annual plant that grows floating on the water surface.
• The seeds are rich in starch and have been used as a food source for centuries.