This annual floating-leaved aquatic herb belongs to the natural order
Trapanaceae, family Trapaceae. The genus Trapa comprises some 30 species that
exclusively grow in eutrophic water. Trapa natans Linn. var. bispinosa
Makino is a native of China. The Chinese name of the nut is Ling Ko, meaning “spiritual horn.”
Ling Ko is found all over China. It is harvested and consumed during the mid-autumn festival in
September to celebrate the overthrow of the Mongolians during the Yuan Dynasty in ancient
China. Trapa bispinosa Roxb. grows in India and also in Ceylon. The nut has two
(sometimes four) short slender spines in place of the pronounced horns of the Chinese plant. The
fruit with two spines is known as Trapa bispinosa Roxb., and the four-spined fruit is
known as Trapa quadrispinosa Wall. There is also a four-spined European species,
Trapa natans, commonly known as Jesuit nut, water caltrops, or water chestnut.
These nuts are of a slate brown color whereas the Chinese nut is black. Based on the color of the
husks, water chestnut is categorized into three types: completely green, completely red, and
green blended with red.
About 2/3 of the water chestnut plant floats just beneath the water surface and thus forms a thick
mat in the water column. Only its upper leaves float over the water surface in an artistic radial
pattern with swollen, air-filled petioles that keep the upper part of the plant afloat. The reddish
green leaves are villous on the dorsal side, and 6-8 cm in size. The submerged leaves (occurring
on young plants and not shown in drawing) are laterally dissected into capillary segments.
Trapa has no primary root. The plant stem remains in the water and has one
node of about 3-5 cm in thickness. The submerged stem bears two types of adventitious root.
Those near the base of the stem fix the plant to the muddy substrate. The rest are free-floating
fibrous roots borne in pairs below the leaf bases and are unusual in being green and
photosynthetic.
The flowers are axillary, white in color, with a solitary peduncle. They open above the
surface of the water towards the afternoon. After pollination, the flowers submerge to facilitate
fruit formation. Fruits appear in September in the State of Bihar, and continue up to December
and January, fully ripening in the cold season.
The plant bears edible nuts in hard-shelled fruits which resemble the head of a water buffalo with its two large curved horns. The fruit has four angles and two out of four develop in the case of Trapa bispinosa. The fruit is a bony one-seeded nut having very unequal cotyledons and a top-shaped drupe. The fleshy pericarp covers a large 2-4 horned, stony endocarp. When ripe, the nuts fall to the bottom of the pond where they remain all winter as they must be kept moist to retain their viability.
Cultivation
Ponds which are otherwise unsuitable for fish culture are being utilized for farming of
this fruit crop. It is best grown in shallow perennial ponds which hold abundant water
throughout the year.
Trapa can germinate under a wide range of water depths and grows best at .5-1 m.
The maximum water depth should never be more than 1 m, though the plant can grow to a depth
of around 3 m. The plant requires full sunlight and the water level should be full by August.
The pond water must have a high organic content and should be free of high concentrations of
salts. Neutral to somewhat alkaline pH are best for proper growth of the plant.
In India, the traditional palm tree toddy collector known as “Pasi” by caste and also the fisher
community are engaged in cultivating and marketing of water chestnut. Two methods are used
in cultivation: natural seeding from previous crops, and preparation and transplanting of
seedlings. After the harvest of the seed crop, disease-free, healthy and large sized fruits are
selected for raising in the nursery. While selecting fruits for seeds, spines of the spinous variety
are cut with sharp knives to prevent damaging the outer shell of seeds during curing and
storage.
Selected seeds are stored only after curing with a special technique. The seeding material should
be kept in large barrels or in earthen pitchers which are filled with freshwater and left
undisturbed for two to three days. Afterwards water is changed daily for at least 5-6 days. This
is one of the most essential operations. The practice is continued until the hard, thick outer skin
of the fruit rots and the loose coating of the seed detaches from the fruits and the thin, stony,
inner coat is visible. The curing of seed material is done at room temperature and is completed
in about 35-40 days. The objective of curing the seeds is to prevent spoilage due to rotting of the
loose outer shell of the fruits.
Seeds so cured can easily be stored in the same earthen pitcher or barrel, but without water and
covered with a moist cloth or gunny bag to provide high humidity and low temperature. These
containers are kept in a cool, shady place and can be stored for up to 3-4 months, without
affecting seed viability. The seed nuts procured from 1/100 of a hectare of a normal crop are
sufficient to raise seedlings for one hectare.
During the months of March-April, just after the seeds have started germinating, they are
broadcast into small nursery ponds or in small, shallow ditches having 45-60 cm of water.
Before broadcasting, the seeds are coated with a layer of soil on the opposite face of the
germinated portion in order to add extra weight on the non-germinated face and to assure that
after broadcasting, in the manner of a shuttle-cock, the seeds will settle at the bottom with the
germinated face up and the coated face down. They also can be manually sown. The stem starts
emerging and gradually spreads out. During the months of June-July, seedlings are lifted from
the nursery pond and transplanted into larger ponds, ditches, or reservoirs. For transplantation,
the uprooted stems are cut into several smaller pieces. Some growers fasten 3-4 seedlings
together in a bunch, which is thrashed into the pond bottom by feet.
Lateral shoots commonly known as suckers can be detached from the main mother seed nut for
transplanting. Single seeded water nuts can develop 20-30 and sometimes even up to 50 such
lateral suckers. Each of these laterally developed suckers may very well be able to send out 5-10
further shoots after transplanting. From sowing to later such formations takes about 40-50 days.
Shoots also arise from the nodes, forming roots and new plants. Thus, within a month or so, the
entire water area gets covered with the luxurious growth of brownish-green leaves.
Fertilizing the pond with urea is a common practice. This is applied at the rate of 40-50 kg/ha of
pond surface area in two installments at fortnightly intervals, with the first dose about 20 days
after transplanting. The application of 40 kg of nitrogen, 40 kg of phosphate, and 60 kg of
potash per hectare produces better results.
Pests of water chestnut include the beetle, Galerucella birmanica, which is reported
to consume up to 40% of the leaf tissue. Insect pests are controlled by shaking the plants
vigorously under water, by hand-picking, and by dusting or spraying exposed parts of the plants.
Snails are another destructive pest, particularly during the later stage of growth. Growers remove
the snails by hand. Rats also eat nuts and vegetative parts of the plant.
Harvesting of fruit is from September/October through December/January. The entire crop is
harvested in four installments at intervals of 8-10 days because the fruits ripen in batches. At the
time of harvesting, the size, softness of the pulp, greenness of color, and easy separation of the
outer hard cover are the most important characteristics taken into consideration. Each fruit is
plucked by hand after lifting the plants from the surface of the water. The plant is then put back
in position for the next batch of fruits to ripen during the 8 day interval. Quantitatively, the
maximum yield is obtained on the second and third installments of harvesting operations.
In traditional culture, the yield from 1 bigha (4 bigha=1 hectare) of pond area, on average, ranges
from 2.4-2.6 quintals (quintal=100 kg. or 220 lbs.). With the application of inorganic fertilizer
and pest control measures, an average yield of 10-12 quintals/bigha has been obtained.
The commercial marketing of water chestnut has not been fully investigated. Water chestnuts
are sold fresh on the pond bank, or in local markets, where prices and profits tend to be low.
Editor’s Note: All Trapa species are prohibited in the state of
Florida. Trapa natans, introduced to New York State in the late 1800s, now infests
sites throughout the northeastern United States. The plant has aggressive growth habits and
forms extensive surface mats, restricting both recreational and commercial uses of infested water
bodies. It is reported that seeds may remain viable for up to twelve years, making eradication of
the plant especially difficult. Research continues on control methods for this species.
The Chinese water chestnut, Eleocharis dulcis, is grown legally and successfully in
Florida as a food crop. Confusion is frequent since both plants share the same common name of
water chestnut.