Water Caltrop Nut

Introduction:

Water caltrop is a floating, annual, aquatic plant that grows in slow moving water. It would most likely be found in farm dams, water features, fish ponds or in ponded and slow moving water bodies near towns. When uncontrolled, water caltrop forms large populations that can create nearly impenetrable mats across wide areas of water, out-competing native plants for sunlight and making the waterways inaccessible to boats. The sharp spines on water caltrop fruit are also a hazard for humans and animals. Two species of the genus Trapa – T. natans and T. bicornis – are referred to as water caltrop. Water caltrop is also known as water chestnut, but is not related to Chinese water chestnut (Eleocharis dulcis), which is cultivated for its edible tubers.World Status:

Water caltrop is native to the warm temperate parts of Eurasia and Africa. In China the seeds are cooked and eaten. The species has been introduced into North America and has become an invasive species in eastern areas of Canada and the United States. Although water caltrop is yet to be found in NSW it has the potential to become a significant weed.

Identification:

Trapa natans is an annual herb with a floating rosette and a stem that is rooted in the soil below the water level.

Stems:

Submerged stems are unbranched and long, reaching 3.6–4.5 m in length. The stems are anchored into the mud by very fine roots.

Leaves:

There are two types of leaves. The first leaves produced are submersed and fall off during early stem growth. The submersed leaves are finely divided and feather-like. The second type of leaves form on the water surface in a rosette. These floating leaves have saw-tooth edges and are oval or triangular shaped, 2–3 cm long, with the upper surface glossy and the lower surface covered with fine short hairs.

Roots:

After the submersed leaves drop off, featherlike roots up to 8 cm long develop in their place. These roots are often mistaken for leaves (see image above).Flower: The flowers form above the water surface in early summer. The four-petaled flower is white and about 8 mm long.

Fruit:

The fruit is a woody or bony nut, about 3 cm wide, with four (1 cm long) stout spines or horns. Each fruit contains a single seed. Trapa bicornis is similar to T. natans. One distinctive feature is that T. bicornis has two spines on the fruit instead of four.

growth and Spread:

Water caltrop is a floating, annual, aquatic plant growing in slow moving water up to 5 m deep. The plant over winter seeds in mud. These seeds germinate during the warmer months producing stems that reach the water surface and produce rosettes. A single seed may give rise to 10 to 15 plant rosettes. Each rosette can produce up to 15 to 20 seeds. The plant spreads when rosettes break away, by fruits detaching from the stem and floating to another area. The fruit can also be spread by birds and other animals. Seeds can remain viable for up to 12 years, although most will germinate in the first two years. If the fruits are left to dry out they die.

control:

Hand removal is an effective means of eradication of smaller populations. Plants are easily hand pulled and should be removed from the waterway to prevent their spread. The potential of Trapa spp. seeds to lay dormant for many years makes total eradication diffcult and continual monitoring necessary. Hand harvesting from canoes and raking have been effective in small outbreaks. Herbicides and mechanical harvesting have both been effective for control of large-scale populations of Trapa spp. in the United States.

The water chestnut, also called the Chinese water chestnut or the water caltrop, is a tuber vegetable that resembles a chestnut in color and shape. Although it is most commonly associated with Chinese cooking, it is now gaining in popularity as a cooking ingredient in many different ethnic meals. Originating in Southeast Asia, water chestnuts are the roots of an aquatic plant that grows in freshwater ponds, marshes and lakes, and in slow-moving rivers and streams. Currently, water chestnuts are grown in Japan, Taiwan, China and Thailand as well as in Australia. When harvesting water chestnuts, much labor is involved. Because of this, water chestnuts are fairly expensive to purchase, especially in a processed or canned form.

However, processed and canned water chestnuts are the most common form used for producing and mass-marketing water chestnuts to consumers. You can easily purchase these products in most supermarkets and grocery stores. You can also purchase fresh water chestnuts, which are more difficult to find, at some specialty vegetable stores and ethnic markets. When purchasing fresh water chestnuts, however, know who your supplier is first to ensure that the tubers, which you are buying, are fresh.

Once you have purchased your water chestnuts, if canned, you can store them unopened in your home pantry. If a can has been opened or if you have purchased fresh water chestnuts, you can store them for up to one week in your refrigerator. However, you must keep the water chestnuts in a bowl of water. The water must also be changed every day in order for them to remain fresh while they are being stored.

Water chestnuts can be used in a variety of recipes because they have a starchy taste that is fairly neutral. Some people claim that their flavor is similar to a bland nut. Water chestnuts also have a firm and crispy texture, which adds to their appeal as an ingredient in stir-fries, salads, or any meals where the vegetables to be used must have a crunchy consistency. Some common uses for the water chestnut include the following:

• combining them with vegetables, such as bamboo shoots and snow peas, then adding soy sauce and other seasoning to make a stir fry

• adding them chopped to soups, salads, rice, and stuffing

• wrapping them whole with bacon then baking the pieces to serve as an appetizer or as a side dish.

Water chestnuts can also be used as a healthy and nutritious snack. For a quick treat, open a can of water chestnuts and rinse the water chestnuts thoroughly under cold water. Dry any remaining water off of the water chestnuts then eat them raw. Water chestnuts are also great for those who are looking for health-conscious vegetable alternatives because they contain little fat as well as provide a good source of fiber and of vitamin B. Additionally, water chestnuts contain a fair amount of the following minerals: calcium, iron, potassium, and zinc. They also offer a flavorful alternative to dieters who are looking to liven-up their meals by using unique and low-fat vegetables and foods. One cup of water chestnuts contains approximately 135 calories.

Water chestnuts will continue to be used as a cooking stable among Asian and international households because of their cooking diversity and because of their unique texture and interesting taste.

acknowledgements:

Dr John Hosking, Elissa van Oosterhout and Rod Ensbey.

Prepared by:

Annie Johnson, Andrew Petroeschevsky and Dr Stephen Johnson.