Invasive Plant Removed in Mill Pond

This article first appeared in the Summer 2012 Sound Update issue and was written by Monica Williams, a Wildlife Biologist at the Long Island NWR Complex in Shirley, NY..

Invasive water chestnut in Mill Pond, Oyster Bay. (Photo by Monica Williams)
Invasive water chestnut in Mill Pond, Oyster Bay. (Photo by Monica Williams)

Water chestnut (Trapa natans) is a highly invasive aquatic plant, native to Europe, Asia, and Africa, and was introduced to North America in the late 1800s. This annual floating plant has green, glossy, triangular leaves, with stems that can grow up to 16 feet in length. Water chestnut produces a black nut with sharp spines, which contains a fleshy seed that forms a white flower in mid-summer. Water chestnut can be found in the northeastern United States, where it invades ponds, lakes, and rivers. It forms dense, floating mats that clog the water in shallow areas and along shorelines. It also limits light and reduces oxygen levels for plants and animals that live in the water column. This non-native, invasive plant competes with native vegetation, is of little value to wildlife, and impacts recreational activities such as boating, fishing and swimming.

One water chestnut seed can produce up to 15 rosettes and each rosette can produce just as many seeds, with the potential to generate 200 or more new seeds in a single year. Seeds can remain viable in the sediment for up to 12 years and, while most seeds germinate within the first two years, management efforts must be conducted for several years to ensure erradication. Water chestnut can spread to new locations naturally by water and birds, but also accidentally by boats, trailers, and fishing gear. Water chestnut recently invaded Mill Pond, an eight acre freshwater pond that drains into Oyster Bay Harbor, NY and is a part of the Oyster Bay National Wildlife Refuge (NWR), which is administered by the US Fish and Wildlife Service. The Oyster Bay NWR is 3,204 acres in size and is unique as a marine refuge consisting primarily of sub-tidal habitats (bay bottom) and intertidal salt marsh, with lesser extents of high marsh and freshwater wetlands. The refuge provides important habitat for marine wildlife, reptiles, amphibians, and more than 100 species of birds.

Between 2005 and 2007, neighbors first noticed water chestnut in Mill Pond and NWR staff confirmed the infestation in 2008. Refuge Staff along with Friends of the Bay, The Nature Conservancy, and several other partners including the North Shore Land Alliance, The Town of Oyster Bay, Huntington/Oyster Bay Audubon Society, local neighbors, and many volunteers, have been working together to remove water chestnut from Mill Pond. Special workdays are conducted in the late spring and summer with staff and volunteers going out on Mill Pond in canoes, kayaks, and small boats to hand pull water chestnut. Plants pulled from the pond are collected in the boats and in baskets and are counted, estimated, and/or weighed. Small amounts of water chestnut that have not set seed are disposed of in an upland area along the western boundary of Mill Pond to decompose, while larger amounts of plant material are transported to the Town of Oyster Bay, Bethpage Landfill. Since water chestnut control efforts began in July 2008, more than 112,214 plants have been removed from Mill Pond with approximately 900 hours of volunteer effort. In 2011, there was a noticeable decrease in water chestnut in Mill Pond. The intensive work done in previous years seemed to have reduced its abundance and distribution. In 2011 and 2012, hand-pulling efforts started earlier in the season (late May) and, with less water chestnut, most plants can be removed from the pond before new seeds are dispersed in July and August. Even though hand-pulling is labor intensive, it was the best control method for this particular infestation as it avoided the use of chemicals and expensive, mechanical equipment. Staff and interns mapped the water chestnut infestations in 2010 and 2011 at various stages during the summer. It is amazing to see how the initial infestation that once covered approximately three quarters of Mill Pond has now been reduced to a few plants along the shore and throughout the pond.

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