The water caltrop (Trapa natans) is sometimes mistakenly called water chestnut. However, the water chestnut (Eleocharis dulcis) is not even closely related to the annual water nut.
Trapa natans belongs to the Lythraceae family. You can find it in moderate to subtropical regions of Europe, Asia, and Africa.
Origin and Distribution
The water caltrop (Trapa natans) is an annual floating leaf plant from the genus of Trapa. The species was already widespread 65 million years ago, in the geological era of the Tertiary and thus at the same time as the dinosaurs.
Today, the water caltrop still thrives in the temperate to subtropical climates of Europe, Asia, and Africa. But it occurs rarely in the wild due to intensive agriculture and dwindling habitats.
Therefore, the species is under strict nature protection in many countries and may not be taken from the wild. However, you can buy legal offspring, which are great for the natural planting of garden ponds.
Only the offspring from Europe are suitable for planting in garden ponds. The tropical varieties of the same name do not find suitable habitats here and therefore do not thrive.
Therefore, always pay attention to the proof of origin when buying! Most offspring of the water caltrop come from Hungary, southern France, and Italy.
You can plant water caltrops individually or in groups, depending on the design preference and available space. However, this floating-leaf plant also harmonizes very well with other native aquatic plants.
Pickerelweed (Pontederia cordata) with beautiful purple flowers, yellow floating heart (Nymphoides peltata), and yellow water-lily (Nuphar lutea) do very well with the water caltrop.
Appearance and Growth
Water caltrops are deciduous, herbaceous aquatic plants that grow for only one summer. Their natural habitat is in standing water, where they are anchored in the muddy bottom near the shore, mainly at a depth of 12-24 inches (30-60 cm).
The submerged stem, between one and three meters long, roots in the lake bottom. The leaves, up to 8 inches (20 cm) in diameter, fan out from June to form a rosette of leaves resting on the water surface.
The submarine leaf stalks of the water caltrop are air-filled and therefore act as floats. They provide the buoyancy necessary to keep the green foliage leaves on the water surface.
The species’ fan- to diamond-shaped floating leaves have a characteristically serrated edge and arrange in rosettes on the water surface.
The leaves turn red during the summer and then die in the fall. Also characteristic are the glands located on the undersides of the leaves as well as on the stem. These probably secrete an acid for protection against hungry aquatic animals.
Flowers and Fruits
The inconspicuous, radially symmetrical flowers of the water caltrop are white and appear between July and August.
The plant’s nut-like fruits form on the stems. They have a hard, dark brown shell, are spiky and angular.
The white kernel of the water caltrop fruit is about 20 percent starch and is edible when cooked. In fact, the nutritious water caltrop was even considered food in earlier times. And it still is, especially in Asian countries.
The white interior of the water caltrop is edible, but you should only ever eat it cooked or roasted. Raw fruits are considered poisonous.
Also, parasites that are dangerous to humans like to settle on the surface. Furthermore, the aroma, which is slightly reminiscent of chestnuts, only develops with cooked fruits.
The hard skin of the fruit is inedible. But you can easily open it with your fingers or a sharp knife and a little effort.
Location and Soil
Water caltrops thrive only in standing water that is warm and sunny. The plants are unsuitable for streams and other flowing waters, and it is also better not to plant them in fish ponds.
Water, as well as the substrate, should be nutrient-rich and low in lime as the water caltrop is only slightly tolerant of lime.
Water caltrops feel most comfortable when the pond water is slightly acidic. You can achieve this by adding pressed peat soil that is available from specialist retailers.
You should plant the water caltrop only in ponds that are between 16 and 24 inches (40 and 60 cm) deep.
Planting Water Caltrop Properly
The easiest way to establish water caltrops on your garden pond is to sow them instead of planting them. You can get the seeds or nuts at garden supply stores.
Simply sink them into the water in the fall, and they will take root in the bottom of the pond all by themselves over the next few months and sprout the following June.
For an average garden pond, calculate about two to three plants, but they should not be settled near the pump.
In the spring, stores sometimes offer water caltrop plants, which you plant out as follows:
- Place the plants on the calm water surface.
- Anchor them in the bottom of the pond with a wire.
The long stems along with the roots will then develop so that after a few weeks the plant will have grown in the garden pond. It will then be able to provide itself with nutrients.
Water caltrops need a sunny spot in a stagnant freshwater pond with a water depth of up to 24 inches (60 cm) and a sandy-muddy substrate. Provided that the site requirements of the water caltrop are met, there are no care measures necessary.
The plant is hardy to temperatures of at least -8 °F (-22 °C).
Once sown, the annual water caltrop practically reproduces itself for years. As soon as the leaf rosette dies in the fall, the nut fruits sink to the bottom of the pond and overwinter there.
In spring, long, thin stems sprout from them and grow towards the water surface. From these, the leaves develop starting in June, eventually resting on the water in a floating rosette.
Pruning Water Caltrop Properly
To avoid contaminating the water, you should cut off the wilted leaves in small garden ponds or aquariums in the fall. In larger ponds, however, this precaution is not necessary.
Propagating Water Caltrop
Selective propagation of water caltrop is neither necessary nor possible. The plant reproduces by itself via the formed fruits, provided that the site conditions meet its requirements.
The nut-like drupes, which are basically nothing more than specialized overwintering organs, sink to the bottom of the pond in the fall and then sprout into new plants the following spring.
Since each water caltrop produces several fruits during the season, a dense carpet can form on the garden pond in this way over time.
For propagation to succeed, you should cultivate water caltrop as the only aquatic plant, because other species would compete for the nutrients present in the water.
Since water caltrops require a high level of nutrients to form their fruits, they would not form fruits and would simply die in that case.
In the fall, you can remove the nuts before they drop. Store them in a container with water until it is time to reapply them. Keep in mind to replace the water regularly.
In any case, do not use tap water for this purpose, because water caltrops do not tolerate lime. Instead, add acidic, compressed peat soil to collected rainwater or similar.
In spring, you can re-sprout the seeds can in warm water and then place them outdoors. However, before planting them out, slowly accustom them to the climatic change so that they do not die from planting shock.
If your pond is already densely stocked with water caltrops, you can quite easily remove partial plants and transplant them to other ponds.
Dividing Water Caltrop
Water caltrops cannot be divided because each nut develops only one floating stem with a rosette of leaves.
Diseases and Pests
Diseases are unknown in water caltrops. And this aquatic plant is also not affected by pests. However, incorrect care or an unsuitable location can be problematic.
Species and Varieties of Water Caltrop
Two varieties of the water caltrop are known. Trapa natans var. natans is available only as a cultivar. The plants, which grow primarily in boggy or swampy areas, may not be taken from the wild. The offspring usually grow well, but do not always form fruits.
The species Trapa natans var. bispinosa, also known as Singhara water caltrop, originated in China. It is also available from specialty stores.
The leaves of this variety are olive green and typically have seven reddish to reddish-brown parallel veins on the leaf blade. The species is not hardy and must therefore overwinter in artificial light in the winter garden or in a greenhouse.