Water caltrop: planting & care of Trapa natans

Water caltrop (Trapa natans) is a very special aquatic plant. We show you how to plant and care for water caltrop in your own pond.

Water caltrop is an endangered beauty that can also decorate your garden pond. Find out below how to cultivate and care for this special aquatic plant. We also show how the fruits can be used.

Water caltrop: origin and properties

Water caltrop (Trapa natans) is botanically classified in the subfamily Trapoideae of the loosestrife family (Lythraceae). Although the names of the water caltrop and the water chestnut (Eleocharis dulcis) sound very similar, the two are not related and are visually very different.

Originally, water caltrop comes from the temperate and subtropical zones of Europe, Asia and North Africa. Today, it is still weakly distributed throughout the Mediterranean area. In Germany, the last stocks are threatened with extinction, which is why the plant has been protected since 1987 and is on the red list. It was an important source of food for people in the Neolithic period, and today it is still occasionally used in Asian cuisine.

The water caltrop plant is a deciduous annual floating plant. It is anchored to the ground by a 1 to 3 metre-long main shoot and by the barbs of the fruit. The herbaceous foliage leaves are held at the water surface by an air-filled stem. They are fanned to diamond-shaped and arranged rosette-like around the petiole. The plants can reach a diameter of about 20 cm. The formation of leaves begins in June. On the underside of the leaves and on the stems there are acid-secreting glands that serve to protect against being eaten. Small, inconspicuous, white flowers are formed from July to late August or early September. Water caltrop is self-pollinating, which means that the flowers can be pollinated by their own pollen. During the autumn, the leaves turn a beautiful red colour and gradually die. Paired secondary roots are formed on the main shoot to help the water caltrop photosynthesise.

The fruits of the water caltrop are dark brown and hard-shelled nuts. They grow along the stem and form two to four pointed spines at the ends. The spines later serve, as with an anchor, to fix it to the bottom of the water. Thus, the next year a new plant can grow without drifting. Inside is a white, starchy core. The kernel is ripe between September and October and germinates the following year if it is warm enough. In addition to being spread by water, the nuts can also be further spread by birds or by humans.

Varieties

It is generally distinguished between two varieties of water caltrop:

• Trapa natans var. natans: the European native Trapa natans var. natans is the water caltrop that grows best in our country. Since it is threatened with extinction, the seeds come exclusively from offspring. Some plants do not form fruits.
• Trapa natans var. bispinosa: the Asian-origin Trapa natans var. bispinosa (also called ‘Chinese two-horned water nut’ or ‘Singhara water nut’) produces olive-green leaves with reddish or reddish-brown veins. Due to the shorter summer in our country, it does not have enough time to form fruits. It needs a tropical climate for this.

Planting water caltrop

The natural habitats of water caltrop include stagnant waters, old waters low in lime but rich in nutrients, humus mud lakes and ponds, and the lower third of the Danube from Serbia. Due to its heat requirement, it does not grow at higher altitudes. If you want to cultivate the plant in your garden pond, you will need a muddy-sandy pond bottom and an average depth of about 60 cm. Also, it should be warm and sunny. Water caltrop prefers slightly acidic pond water, but it is not a must.

To sow water caltrop in the pond, it is enough to place the nuts near the desired place on the bottom of the pond. The plant then anchors itself and grows. For smaller ponds, as few as 2 to 3 nuts are enough to achieve a beautiful planting appearance. Group planting is also possible without further ado. Yellow pond lily (Nuphar lutea), European pondweed (Nympoides peltata) and heartleaf pikeweed (Pontederia cordata) are particularly well suited for this purpose.

Note: Planting in a fish pond is less suitable because of the acid that is secreted by the plant as protection against feeding. Nevertheless, this is possible without endangering the fish. However, water caltrops sprout only once, so the plants are very sensitive to feeding damage and may die.

Water caltrop reproduces by itself through the formed nuts. The ripe fruits fall off as soon as the annual plant dies and spread through the water. They are frost-resistant and hardy down to temperatures of -22 °C. The following year, they anchor themselves in the ground and sprout again on their own. If you want to specifically propagate water caltrop, you can harvest ripe nuts and sow them the following year. Propagation via cuttings, comparable with cuttings propagation in woody plants, is not possible.

Tip: Water caltrop can also be kept as a single plant in an aquarium. Here, an ideal nutrient supply can be ensured and there is no competition from other plants. However, the plant does not form fruits in the aquarium. If you want to keep fish in the aquarium at the same time, be sure to check their compatibility with the acidity of the water caltrop plant.

Caring for water caltrop

When planted in a location that meets its needs, water caltrop does not require further care throughout the year. For small ponds and in aquariums, some to all leaves should be removed in the autumn as soon as the plant dies, depending on the size of the pond or aquarium. This way, the water is not overly polluted with decaying biomass. There are no problems with this in large ponds.

Since it is an annual plant that dies in the autumn, you cannot overwinter water caltrop as a whole plant, but only as seeds. To do this, the seed can be fished out of the pond and overwintered in a bowl of water or simply left in the pond.

Harvest and use

The ripe fruits of water caltrop can be easily harvested when the plant dies. They can then be detached from the main shoot and collected.

You can eat the fruit kernels of water caltrop. As mentioned at the beginning, water caltrop was an important source of protein in the Neolithic period. In Asia, it is still occasionally used today. However, it has almost completely disappeared from European cuisine because of its rarity. Raw consumption is strongly discouraged, as numerous harmful parasites can inhabit the surface. By cooking the caltrop, you break down the toxins it contains and kill parasites. The core is then edible but the shell remains inedible due to its hardness. The aroma of the core is comparable to sweet chestnuts.
In addition, water caltrop is said to have a healing effect but this has not yet been proven.