Invasive Species of Concern in Maryland

This past summer, US Geological Survey scientists discovered the exotic plant water lettuce (Pistia stratiotes) floating over submerged aquatic vegetation beds in Mattawoman Creek, a large Potomac River tributary in Charles County. They raised an alarm in the aquatic invasive species community, because water lettuce can form dense carpets of vegetation on the water surface, blocking sunlight from reaching submerged plants and reducing the oxygen exchange at the water’s surface. It can also grow to form surface mats impenetrable to boats, swimmers and waterfowl. Although water lettuce is a perennial plant, it would not normally survive Maryland’s winter temperatures, because it has a low temperature minimum of 59°F for growth. Yet it has been found as far north as New York. Questions still exist about its origin, its ability to withstand northern winters, its spread rate and the effects of rising water temperatures on its possible spread north. For this reason, the Maryland Invasive Species Council has named water lettuce the January Invader of the Month.

Water lettuce is a worldwide aquatic weed that floats on the surface of slow-moving rivers, lakes and ponds. It thrives in tropical and subtropical regions. Some people consider it native to the US because the botanist William Bartram discovered it in Florida in the mid-18th century. Generally, however, water lettuce is thought to originate in South America, because the widest suite of native aquatic insects associated with it occurs there.

Many aquatic invasive plants look alike, but water lettuce is distinctive and almost impossible to mistake, as it looks like an open head of lettuce. It has a cluster of soft, light green hairy leaves with parallel ridges (leaf veins), grouped in a rosette. It draws nourishment from the water column, with a short bunch of feathery roots hanging below it. It reproduces both by seed, and vegetatively, with daughter plants connected to mother plants by short stolons. It is this vegetative growth that is responsible for the plant’s rapid spread as surface mats. It is not known whether water lettuce seed lives through the winter in Maryland and germinates into new plants in the spring. Water lettuce can survive in mud, but prefers water. Its flowers are very small and inconspicuous, hidden in the center of the rosette, with male flowers above a single female flower on the same fleshy stalk. The fruit is a many seeded green berry.

Mechanical, chemical and biological controls exist for the water lettuce. The plant can be raked or seined from the surface. Spread out in a layer away from the water, the plant will desiccate and die. Non-selective herbicides such as glyphosate have been used effectively on water lettuce, as the leaves stand up from the water surface and provide a target for direct application. Non-selective contact herbicides that kill or desiccate foliage rapidly, such as endothall and diquat, have also been used successfully on water lettuce. Two biological control agents, the water lettuce leaf weevil (Neohydronomus affinis) and water lettuce leaf moth (Spodoptera pectinicornis) have been effective on water lettuce in various parts of the world. Both adults and larvae of the weevil, which comes from South America, feed on the leaves of water lettuce. First tested in Australia, these weevils were tested in Texas in the 1990s. Both Florida and Texas have imported the moth from Thailand to combat water lettuce; the larvae can inflict significant damage on water lettuce. We are not aware of any herbivores that eat water lettuce in Maryland.

For more information about other Invasive Species of Concern, visit www.mdinvasivesp.org or call the Maryland Department of Agriculture at 410-841-5920.