Water Chestnut

Introduction

Water chestnut (Trapa natans) is an invasive aquatic weed that can take over and dominate ponds, shallow lakes, and river margins. Water chestnut which also goes by the picturesque local names of "devil nut", "death flower", and "water caltrop" was first introduced to North America in 1886 by a priest named J.H Wibbe who planted it for ornamental reasons in Sanders Pond located in Schenectady New York.(12) Since then the water chestnut has spread to many other areas in the north eastern United states including Connecticut, Delaware, Maryland, Massachusetts, New Hampshire, Pennsylvania, Vermont, Virginia, and Washington D.C. In the summer of 2006 the water chestnut was also discovered in Goodyear Lake in Milford New York. Figure 1 below shows the current distribution of water chestnut in the United States. (2)

(Figure 1) - SOURCE**^*%^&*%&^$%

The chestnut is native to Europe, Asia and Africa and grows best in shallow, nutrient-rich lakes and rivers with a pH range of 6.7 to 8.2 and alkalinity of 12 to 128 mg/L of calcium carbonate.(2)Interestingly the chestnut which has become such a nuisance in America is an endangered species through much of Europe.

Water Chestnut is an annual fresh water aquatic plant with a submerged flexible stem that anchors into the mud and extends upward to the surface of the water. At the surface, the terminus of the plant contains a rosette of floating leaves. The serrated leaves are triangular in shape and connect to an inflated petiole. The inflated petiole or bladder is not hollow but contains the same cellular, spongy texture as the petiole and provides added buoyancy for the leafy portion of the plant.(4) The inflated bladders are approximately 15 cm long, and the leaves are found to be 2 to 4 cm long. These bladders are significant because if a plant becomes detached from its root system it can float away and allow for further dispersal.

Additional feather-like leaves can be found along the submerged stem. These leaves have been described both as roots and as leaves and are structurally similar to adventitious roots hairs. They serve primarily in nutrient absorption but can be photosynthetic.(9) Trapa natans like many aquatic plants has no primary root system but rather posses adventitious roots that anchor the stem at one or more locations to the muddy substrate.

The plant contains a small white flower which develops in late June from the axis of younger leaves above the water. It possesses four petals which are approximately 8 mm long and are entomophilous (insect pollinated).

The water chestnut develops a caltrop shaped nut or fruit that has four half-inch barbed spines. These spines seem to be of use in anchoring the nut among the debris on the water bottom. At maturity, the nut breaks off at the peduncle attachment and sinks to the bottom. A Trapa nut is 20% heavier then the water it displaces, so drifting in the water column of nuts is rarely a means of dispersal.(12) Figure 2 below diagrams the parts that make up the water chestnut.

Figure 2- SOURCE^%#^%#&^#&^#

Dispersal of Trapa nuts can take place by several different means. Occasionally mature seeds are produced by broken off rosettes which can drift considerable distances. Canada geese are suspected of being able to transport the seeds in their plumage. Also the seeds could be moved on boats, bait buckets or other human means. (10)

Seeds over winter at the bottom of the water body and germinate during and throughout the spring and summer to produce shoots that grow to water surface where the typical rosette is formed. Seeds can remain viable for up to five years which makes permanent eradication very difficult.

Each chestnut seed can potentially create 15-20 rosettes. Each rosette can generate up to 20 seeds. This means that this extremely prolific plant can dominate a water body extremely quickly (2). Where water chestnuts are present they can grow in density of up to 50 rosettes per 1 m2 which enables it to cover the water with up to three layers of leaves.(5) The result is that water chestnut shades out native plants such as native wild-celery (Vallisneria americana) and clasping pondweed (Potamogeton perfoliatus) who are unable to compete with the invasive.(10) Also, the enormous amount of plant matter the chestnut creates, drops to the bottom of the water column each winter and further exacerbates the problems of eutrification.

In addition to the ecological problems caused by the water chestnut it creates considerable economic damages. When the plant occupies a site, most recreational activities such as swimming, fishing, and use of small boats are eliminated or severely impeded. The primary economic costs related to T. natans are associated with the costs of chemical and mechanical control efforts. Several different control methods have been developed to eliminate the water chestnut including: mechanical, chemical, and biological.

Mechanical control methods include hand pulling and use of mechanized cutter boats. Hand pulling is very labor intensive and requires large amounts of volunteer support but works best in very shallow waters. Care must be taken to pull the plants early in the summer before seeds drop. Mechanized cutter boats are expensive but are used extensively on large water bodies such as Lake Champlain. Pulled plants can be deposited on shore where they quickly rot.

The herbicide 2,4-D has historically been used to control water chestnut. However the maximum application of 2,4-D which federal regulation permit in water bodies is ineffective against the water chestnut. Moreover, herbicides can be hazardous to more valued plants, microbial communities and young fish populations. Several rare plants disappeared from the tidal Hudson between the 1930's and 1970's while 2,4-D application was in use. The disappearance of these plants including Nuttall's micranthemum (Micranthemum micranthemiodes) could be the result of attributed 2,4-D use.

Research has been done into weather water chestnuts can be controlled via biological means. Numerous