ANNAPOLIS, MD (January 3, 2008)
Potomac Riverís Floating Salad Bar Has No Takers
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 waterís 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 Marylandís winter temperatures, because it has a low temperature
minimum of 59įF 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 plantís 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.