The water nut (Trapa natans) is sometimes incorrectly called the water chestnut. However, this one - Eleocharis dulcis - is not closely related to the annual water nut. Trapa natans belongs to the loosestrife family and occurs in the temperate to subtropical regions of Europe, Asia and Africa. The aquatic plant, which used to be frequently found in Germany, is now threatened with extinction in this country and was therefore placed under nature protection in 1987.
What is a water nut?
The water nut (Trapa natans) is an annual, herbaceous aquatic plant that occurs in stagnant water in temperate to subtropical regions. It produces rosette-shaped floating leaves and inconspicuous white flowers. Their edible fruits are reminiscent of chestnuts and were once a food item.
Origin and distribution
The water nut (bot. Trapa natans) is an annual growing floating leaf plant from the genus of water nut family (bot. Trapaceae). The species was already widespread 65 million years ago, in the Tertiary geological period and thus at the same time as the dinosaurs. Today the water nut still thrives in the temperate to subtropical climate zones of Europe, Asia and Africa, but is rarely found wild in this country due to intensive agriculture and dwindling habitats. That's why the species is under strict conservation protection and may not be taken from the wild. However, you can get legal offspring in stores that are ideal for natural planting in garden ponds.
Usage
Only offspring from Europe are suitable for planting in your home garden pond, as the tropical varieties of the same name do not have suitable habitats here and therefore do not thrive. Therefore, always pay attention to the proof of origin when purchasing! Most of the offspring of the water nut come from Hungary, southern France and Italy.
Water nuts can be planted individually or in groups, depending on the design wishes and available space. The floating leaf plant also harmonizes very well with other native aquatic plants such as the heart-leaved pikeweed (bot. Pontederia cordata, violet flowers), the European sea jug (bot. Nymphoides peltata, yellow flowers) and the yellow pond lily (bot. Nuphar lutea).
Appearance and growth
Water nuts are deciduous, herbaceous aquatic plants that only grow over one summer. Their natural habitat is in stagnant waters, where they are anchored in the muddy bottom near the shore, primarily in 30 to 60 centimeters of water. The submerged stem, which is between one and three meters long, is rooted in the lake bed, and the leaves, which are up to 20 centimeters wide in diameter, fan out from June to form a leaf rosette lying on the water surface.
leaves
The undersea petioles of the water nut are filled with air and therefore act as floating bodies. They provide the necessary buoyancy that keeps the green leaves on the surface of the water. The fan-shaped to diamond-shaped floating leaves of the species have a characteristic jagged edge and are arranged in a rosette shape on the water surface. The leaves turn red over the summer and then die back in the fall. Also characteristic are the glands on the underside of the leaves and the stem, which presumably secrete an acid to protect against hungry aquatic animals.
Flowers and fruits
The inconspicuous, radially symmetrical flowers of the water nut 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 sharply thorny and angular. The white core of the water nut fruit consists of around 20 percent starch and is edible when cooked. In fact, the nutritious water nut was also considered food here in earlier times and is still so today, especially in Asian countries.
Toxicity
The white interior of the water nut is edible, but should only be eaten boiled or roasted. Raw fruits are considered poisonous, and parasites that are dangerous to humans tend to settle on the surface. Furthermore, the aroma, which is slightly reminiscent of chestnuts, only develops during cooking. The hard shell of the fruit is inedible, but can be easily opened with your fingers or a sharp knife and a little effort.
Location and soil
Water nuts only thrive in stagnant water that is warm and sunny. The plants are unsuitable for streams and other flowing water, and you should also not plant them in fish ponds. The water and the subsoil should be nutrient-rich and low in lime - the water nut has little lime tolerance. Water nuts feel most comfortable when the pond water is slightly acidic. You can achieve this by adding pressed peat soil (€8.00 on Amazon). You can get these from specialist retailers. Only plant the plant in ponds that are between 40 and 60 centimeters deep.
Planting water nuts correctly
The easiest way to grow water nuts on your garden pond is to sow them instead of planting them. You can get the seeds – the nuts already described – from specialist garden shops. Simply immerse them in the water in autumn; they will root themselves in the bottom of the pond over the next few months and sprout in the following June. For an average garden pond you can expect around two to three plants, but these should not be planted near the pump.
In spring, specialist retailers sometimes offer water nut plants that you can plant as follows:
- Place the plants on the calm water surface.
- Anchor them in the bottom of the pond with a wire.
The long stems including the roots then develop so that after a few weeks the plant has grown in the garden pond and can supply itself with nutrients.
Care tips
If the water nut's location requirements - a sunny spot in a stagnant freshwater pond with a water depth of up to 60 centimeters and a sandy-muddy substrate - are met, any care measures are not necessary. The plant is hardy down to temperatures of at least minus 22 degrees Celsius.
Once sown, the annual water nut practically reproduces itself over the years. As soon as the leaf rosette dies in autumn, the nut fruits sink to the bottom of the pond and overwinter there. In spring, long, thin stems emerge from them and grow towards the water surface. From June onwards, the leaves develop and ultimately lie on the water in a floating rosette.
Cut the water nut correctly
In order not to contaminate the water, you should cut off the withered leaves in small garden ponds or aquariums in autumn. In larger ponds, however, this precaution is not necessary.
Propagate water nuts
Specifically propagating the water nut is neither necessary nor possible. The plant reproduces on its own via the developed fruits, provided 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 autumn and then sprout into new plants the following spring. Since each water nut produces several fruits during the season, a dense carpet can form on the garden pond over time. In order for propagation to be successful, you should cultivate the water nut as the only aquatic plant, because other species would be competitors for the nutrients present in the water. However, since water nuts need a lot of nutrients to develop their fruits, if they were to be planted further in the pond and the resulting drop in the nutrient level, they would not produce any fruits and would simply die.
In autumn, the nuts can be removed before they sink. Keep them in a container of water until you release them again, changing it regularly. Under no circumstances should you use tap water for this, as water nuts do not tolerate lime. Instead, add acidic, pressed peat soil (€8.00 on Amazon) to collected rainwater or similar. In spring, the seeds can be pre-germinated in warm water and can then be placed outdoors - but before planting out, slowly get them used to the change in climate so that they do not die from plant shock.
If the pond is already densely populated with water nuts, you can easily remove partial plants and transplant them into other ponds.
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Water nuts cannot be divided because each nut only develops a floating stem with a rosette of leaves.
Diseases and pests
Diseases are unknown in water nuts, and the aquatic plant is not affected by pests. However, care errors or an unsuitable location are problematic.
Tip
Water nuts can also be cultivated very well in a water basin in the house - for example in the winter garden or in a large aquarium. However, fish are not allowed to swim in this container. You also need to provide the necessary lighting using artificial light (e.g. LED plant lamps).
Species and varieties
There are two known varieties of water nut. The Trapa natans var. natans, which is also native to us, is legally only available as a captive breed; you are not allowed to remove the plants from the wild, which primarily grow in boggy or swampy areas. The offspring from Italy, Hungary and southern France available in this country usually grow well, but do not always produce fruit.
The species Trapa natans var. bispinosa, which comes from China and is also known as the Singhara water nut or Chinese two-thorned water nut, is also available from specialist retailers. The leaves of this variety are olive green and typically have seven reddish to reddish brown colored parallel veins on the leaf blade. The species is not hardy in our area and must therefore be overwintered with artificial light in the winter garden or in a greenhouse.
