The tiny plant commonly known as duckweed can be prolific and cover a pond, swamp, or even a lake with a green mat, and just within a summer! Left photo shows a slough with the surface covered by the tiniest of duckweeds, known as Wolffia. Wind action can cause a thick accumulation of these tiny plants along the bank.
Three kinds of common duckweed: from top, Spirodela , Lemna, and Wolffia. The scales above each image show spaces of 1 mm between bars. Three genera are common in Arkansas. The underside of the leaves of Spirodela is purple, and it is the larger species of duckweed at about five mm in length. The common duckweed (Lemna) is smaller at about three mm. Known as "water meal" due to its tiny size, Wolffia is less than one mm long, and its fruit is less than 0.5 mm in width. Most reproduction of duckweed is done vegetatively, but these are also the world’s smallest flowering plants. A microscope may be needed to view their flowers well. With the exception of Wolffia, duckweed has roots. Roots tend to be longer when nutrients such as nitrogen and phosphate are low, and shorter when the nutrients are abundant. However, most nutrients are absorbed through the lower surface of the frond (the name for the leaf). The long roots of several fronds become entangled with each other, which apparently increases stability against turbulence by wind and water. This keeps the undersurface of the frond in contact with the water, so the limited nutrients can be absorbed more efficiently. Duckweed may cover a body of water during summer but die back during a freeze. Some kinds of duckweed may regrow from seeds, and others may survive by forming turions. Turions are vegetative buds that stay dormant (they don’t grow) during winter. They are heavier than water, so they sink and overwinter in the mud and debris at the bottom of the pond.
Farm ponds may become undesirably covered with duckweed. One cause of such heavy growth is the excess fertility of a pond, which is increased by runoff of agricultural fertilizers or from animal wastes in pastures. Control of duckweed by herbicides adds chemical pollutants to the environment. Because duckweeds are monocots, they do not respond well to broadleaf herbicides. Biological controls, such as adding herbivorous species of fishes to the aquatic system, may be too slow. Further, the species of fishes that might serve as biological controls often are not native, which means the effort would add new species to an ecosystem, and that can cause several other undesirable effects on the environment.
Mechanical removal of the floating mat of duckweed by skimming the surface can get the job done quickly, at least in smaller ponds. In all treatments, duckweed likely will return because it is difficult to remove every plant. Even if all plants were removed, migrating waterfowl and turtles would reintroduce them eventually. On the positive side, the mats of skimmed duckweed contain a lot of edible proteins. They can be fed to poultry, fish, or cattle, and the duckweed can be composted and used as fertilizer.