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European water-chestnut
NATIVE RANGE:
Eurasia
Leaves - The
conspicuously toothed leaf blades are ½-1 inch long and rhombic in outline
(with four sides); the spongy, inflated leaf stalks are up to 3 inches long and
provide the buoyancy to keep the terminal leafy portion of the plant floating
at the water surface. Green, feather-like, submersed leaves (considered by some
to be modified roots) with very fine segments are present on the underwater
portion of the stem.
REPRODUCTION AND METHODS OF DISPERSAL: In our climate European water-chestnut is an annual. Both vegetative reproduction and seed production take place. Flowers are produced singly on stalks arising from the leaf axils. Each flower is bisexual, bearing a two-chambered ovary, four stamens and four white petals. Once the ovules of the insect-pollinated flowers are fertilized, the flower stalks curve downward with the result that the fruits develop under water. The ovary and surrounding sepals mature into a nut-like, barbed spiny fruit. Flowering begins in the northeastern United States in July and fruits ripen in about a month. The plants continue to flower until killed by frost. One acre of water chestnut can produce enough seeds to cover 100 acres the following year. Each seed can give rise to 10-15 rosettes, and each rosette may produce as many as 20 seeds. Seeds have been known to remain viable for up to 12 years. The fruits may be dispersed when individual plants are uprooted and float downstream. Fruits fall to the bottom of the water body in the autumn and the seeds overwinter just as in terrestrial annual species. Seeds germinate in the spring, with the young root (radicle) perforating the top of the fruit. The young plants develop narrow, opposite leaves initially. As the plant matures, the typical floating, leafy rosettes form at the end of the expanding stem. As soon as the initial shoot develops floating leaves, additional leafy offshoots are produced at a rapid rate. Some of these may become detached and develop into separate plants. It has been suggested that in warmer climates some plants may persist as short-lived perennials. DISTRIBUTION AND HABITAT: European water chestnut is native to tropical and warm temperate regions of Eurasia. It has become naturalized in Australia and in northeastern North America. The Waterchestnut Family contains only a single genus and, depending on the view of the specialist, one, three, or as many as thirty species. This discrepancy is based in large part on the highly variable shape of the fruits. Because the fruits have been used as a source of food and for their medicinal and reputed magical properties, the plants became widely distributed throughout Europe. The plants apparently were introduced into Switzerland during the period of the lake dwellings (around 3000 BC). In the Americas, European water-chestnut is an invasive species known for its aggressive growth habits. Plants were first introduced to North America in about 1874 and were known to be cultured in 1877 in the botanical garden of Asa Gray, the eminent Harvard University botanist. By 1879, plants were found in the Charles River in Massachusetts. Wild populations have since become established in many locations in the northeastern United States. Presently the species is found in Maryland, Massachusetts, New York, Pennsylvania and Vermont. In 1998, it was reported for the first time in Canada, in southwestern Quebec. While not yet widespread in Pennsylvania, it is known from scattered sites in the eastern part of the state. To help control the spread of European water-chestnut, the sale of all species of Trapa is banned in most of the southern United States. Plants grow in quiet streams, ponds, freshwater regions of estuaries, and on exposed mud flats. The widespread occurrence of water-chestnut in aquatic habitats of the northeastern United States is in marked contrast with its present status as a rare species in a number of European countries. It appears to be vanishing in portions of its range in such countries as Belgium, Holland, and Sweden. It is red listed in Baden-Württemberg (southern Germany) and is listed on Appendix I of the Bern Convention (Council of Europe, Convention on the Conservation of European Wildlife and Natural Habitats) as a strictly protected species. EFFECTS OF INVASION: The plants have become an aquatic nuisance species in North America because of their ability to reproduce rapidly and form extensive floating mats. Due to its dense growth, the species impedes navigation and its low food value for wildlife potentially can have a substantial impact on the use of an area by waterfowl and other native species. The dense surface mats likely also inhibit the growth of other aquatic plant species. Decomposition of the abundant detritus produced in the fall of each year as the plants senesce, couldcontribute to lower oxygen levels in shallow waters and thus impact other aquatic organisms. With four, hard, half-inch spines that are sharp enough to penetrate shoe leather and large enough to keep people off beaches, the seeds are a major hazard to water-based recreation.
AUTHORS: Ann F. Rhoads and Timothy A. Block, Morris Arboretum, University of Pennsylvania. For more information
on European water-chestnut, please contact: REFERENCES:
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