Water Chestnut Rosette Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
European water chestnut (Trapa natans), an invasive aquatic plant inadvertently released into waters of the Northeast that is spreading throughout New England and the Mid-Atlantic States, including Pennsylvania, clogging waterways and ponds and altering aquatic habitats. The water chestnut's native range includes Europe, Asia, and Africa. It was first observed in North America near Concord, Massachusetts in 1859. In its native habitat, the plant is kept in check by native insect parasites.
Water chestnut was brought to the United States by water gardeners. It reproduces rapidly; producing up to 15 nuts, each containing a single seed per season. The nuts have sharp spines that can also get caught on other objects, birds, and animals. Trapa can also spread vegetatively. The floating leaves break into fragments attaching to watercraft, or floating to new areas.
European water chestnut is rooted aquatic plant with both submersed and floating leaves. The submersed leaves form feathery whorls around the stem. Floating leaves are glossy green, triangular with toothed edges and forming rosettes around the end of the stem. Single small white flowers with 4 petals sprout in the center of the rosette. The cord-like stems are spongy and buoyant, up to 16 feet in length (although typically in the six to eight foot range).
Water chestnut begins to flower and form seed in mid-July continuing into the fall until frost kills the floating rosettes. Each nut that sinks to the bottom can produce a new plant. Seeds may remain viable for up to 12 years, although most germinate within the first two years. Water chestnut can grow in any freshwater setting; but prefers slow-moving, nutrient rich waters less than 16 feet deep.
Water chestnut is an annual that dies back at the end of each growing season. Seeds germinate in the spring. Hard, nut-like seeds begin to form in July. The inch to inch and a half wide fruits grow under water and have four very sharp spines, capable of piercing footwear. Plants overwinter as seed. Old nuts that are black in color and float are not viable.
Dense floating mats of water chestnut can choke a water body limiting light and oxygen. The colonies alter habitat and out compete native organisms for nutrients and space and can completely dominate an aquatic ecosystem. Water chestnut offers little nutritional value compared with beneficial native plants. These infestations can clog waterways and make fishing, boating, and swimming nearly impossible. Water chestnut is also difficult and expensive to control. The primary economic costs are associated with chemical and mechanical control efforts. For example, Vermont spent nearly $500,000 in 2000 to remove water chestnut using mechanical harvesters and hand removal.
Since water chestnut is an annual plant, control requires preventing plants from blooming and setting seed. Care must be taken during removal because the fragments can form a new plant. A combination of manual, mechanical, and chemical techniques is often the most effective. Larger infestations require the use of mechanical harvesters or the application of aquatic herbicides and infested waters may need to be monitored for 5-12 years to eliminate the invader, and total eradication may never be achieved. The key to water chestnut control is early detection. It is important to spot small populations while they are easy to remove by hand. If you see water chestnut, pull it out and dispose of it far away from the water. Any plant destroyed will prevent up to 120 new plants from growing the next year!