Invasive Pond Plants

Purple Loosestrife

• Purple loosestrife (Lythrum salicaria) is an aquatic perennial native to Asia and Europe. Nineteenth-century immigrants may have introduced it to America as an ornamental or for its medicinal properties, according to the U.S. Department of Agriculture Natural Resources Conservation Services website. As of late 2010, however, 33 states have declared it a noxious weed or otherwise restricted its propagation. Purple loosestrife grows most abundantly along northeastern and Mid-western cattail marshes, bogs, pond and lake shores, stream banks and sedge meadows. Loosestrife colonies may extend over thousands of acres. Eliminating native plants and open water habitat, they endanger bird, butterfly, fish and frog populations. The blooming plants, with their spires of brilliant magenta, mid-summer to fall flowers, make splendid displays. A single flowering plant, however, can produce and disperse more than 2 million seeds.

Waterthyme

• Waterthyme (Hydrilla verticillata) deemed noxious weed, or prohibited or restricted in 18 states as of late 2010, may have arrived Florida in the 1950s for use in aquariums. Over the next two decades, waterthyme spread throughout the state, notes the University of Florida Center for Aquatic and Invasive Plants. It arrived in the mid-Atlantic's Potomac River Area by the 1980s. This submersed plant has mud-rooted tubers and up-to-25-foot stems. The ascending stems cover the water's surface with dense mats of pointed, strap-like leaves. Each tuber can survive for more than four years, producing up to 6,000 new tubers. Plants also spread by stem fragments carried on boats. Waterthyme invades nearly any freshwater habitat up to 20 feet deep. It also tolerates water with 7 percent salinity, and needs only 1 percent of available sunlight to grow. Infestations of this plant change water oxygen levels, eliminating native plants. They also block flood-control and irrigation canals, and damage water pumping facilities. Despite its noxious weed classification, waterthyme is still available for sale.

Water Chestnut

• Native to Asia, Africa and Europe, 11 states list water chestnut (Trapa natans) as a noxious weed, or ban or quarantine it, as of late 2010. Not the same plant as the water chestnut common in Chinese cuisine, this plant gained notice in Concord, Massachusetts in the mid-1800s. How it arrived in America isn't known. It thrives in any freshwater location. Like waterthyme, water chestnut forms light-blocking mats on the waters' surface, interfering with photosynthesis in underwater plants. It also harms fish populations by reducing water oxygen levels. Rooted in the mud, its 12- to 15-foot stems have feathery, submerged leaves and rosettes of triangular, saw-toothed surface leaves. Their 4-inch, white June flowers give way to sharply barbed seed capsules. The capsules attach to birds or floating objects for transport to new areas, where the seeds can survive for up to 12 years, according to the National Park Service.