Deep summer signals the time to be on the water. Whether you're boating or swimming, the lakes and ponds of the Adirondacks have few aquatic equivalents in the country for freshwater enthusiasts. In recent years Adirondack waters have been under a growing siege by aquatic invasive species. Headlines about this summer's AIS included news of didymo, spiny waterflea and milfoil. Another species is leading a quiet march into the region: water chestnut. A single plant may not seem threatening, starting as a small, floating rosette of leaves; but, over time, a multitude of plants numbering in the thousands will result in acres of plants that create a virtual carpet on the water's surface, halting access to boating, swimming, and fishing.
Water chestnut (Trapa natans) is a Eurasian species, which is different than the Chinese water chestnut (Eleocharis dulcis) used in cooking. The Eurasian species was first introduced to the U.S. in 1850s as an ornamental planting in a pond near Cambridge, Mass. The plant has since spread throughout the Northeast, plaguing at least 80 waters in New York.
Unlike other invasive aquatic plants, water chestnut is relatively easy to distinguish from native aquatic plants. It floats on the surface, similar to native water lilies, but its dark green, shiny leaves are triangular and toothed and grow in a rosette. Air bladders on the petioles of the leaves help keep the plant buoyant. Roots of water chestnut anchor in the sediment, sending long stems up from depths of 15 feet to the surface where the leaves float.
The rosette of water chestnut leaves floats on the surface of the water.
(Photo courtesy of APIPP)
The impacts from water chestnut infestations are staggering. The mats of leaves grow so thickly that light readings under dense infestations are equal to that of night. Oxygen levels, too, are so low that conditions stress organisms and compromise habitat for fish and invertebrates. Plant beds are virtually devoid of native plants. The worst infestations severely impair recreational opportunities and cost hundreds of thousands of dollars per year per lake to control.
There is hope, however. Because water chestnut is an annual plant, efforts to eradicate the plant result in greater success than with perennial plant invaders, especially if populations are detected early. For infestations that are small, in shallow areas or intermixed among shoreline vegetation, volunteers use kayaks and canoes, plucking plants and putting them into mesh bags and then composting at neighboring farms. For infestations in larger, open-water areas, mechanical harvesters are used to collect floating plants en mass, which are then composted.
It is critical to harvest plants before they produce seeds, also referred to as nutlets. One plant produces up to 20 nutlets that will sink and settle in the sediment. Each seed then produces 10 to 15 new plants. The seeds aid identification, looking something like from medieval times - prism shaped with four barbed spines; they are also unforgettable and are painful if stepped on.
The nutlets are one of the main methods of spread. The spines stick to carpets of boat trailers and get lodged in wheel wells. Water stewards stationed at boat launches report an increasing number of nutlets being found on boats entering waters across the region. This is cause for concern.
Water chestnut infestations are extensive in Lake Champlain and Lake Ontario. Divers eradicated an infestation from Lake George in the '70s, and small populations continue to be managed in Hadlock Pond in the southeastern Adirondacks.
Otherwise, water chestnut is not known elsewhere in the region. It's essential to keep it that way and hold the line of this aggressive aquatic invader.
Keep your eyes open out on the waterways and report any water chestnut sightings to the Adirondack Park Invasive Plant Program at 518-576-2082.
Eye on Invasives is a biweekly column that spotlights a top invader when it is easiest to identify. Hilary Smith directs the Adirondack Park Invasive Plant Program, a partnership program housed at the Adirondack Chapter of The Nature Conservancy in Keene Valley, N.Y. Find out more about this award-winning program online at www.adkinvasives.com.