Clemson Extension Agents get many calls each year from landowners having problems with aquatic weeds in their ponds. One of the more common plants we receive calls about is duckweed (Lemna spp.). Another plant worth mentioning here because of its similarity in appearance and habit to duckweed is watermeal (Wolffia spp.). Both are floating plants that are among the smallest flowering plants in the world. While they are native species, they can proliferate rapidly and quickly cover pond surfaces, especially in nutrient-rich waters with little airflow. Individual duckweed plants are small, about the diameter of a pencil eraser, while watermeal plants may be about the size of a pencil tip.
How do duckweed and watermeal get established in ponds? Plants that small can easily be transported from one body of water to another on waterfowl, livestock, jon boats, boat trailers, or fishermen’s gear and boots. They may also wash in from one body of water to another during floods.
So, what problems do duckweed and watermeal pose to a pond? From the standpoint of a landowner managing a recreational pond for fishing, there are several. First and most readily demonstrated, it makes it at the least aggravating and at the most impossible to cast and retrieve a fishing lure or bait without your line and reel getting fouled beyond use. Secondly, when most of the surface of a pond is covered, the pond water beneath is deprived of two things, sunlight available to phytoplankton below the surface and oxygenation, both from the air above the pond surface and that produced by phytoplankton and submerged vegetation. Phytoplankton are microscopic algae that give water a green cast, and they form the base of the pond food chain that supports a fish population. There is a continual exchange of gases between the open surface of the pond and the air above. The exchange rate is increased when the wind creates riffles on the surface. Oxygen is also added to the water during photosynthesis by phytoplankton and submerged plants. Duckweed and watermeal release their oxygen directly to the air and not the water, so if either or both form a solid covering of the pond surface, the amount of oxygen making it to the water from the atmosphere decreases. Also, because they intercept available sunlight, phytoplankton below the surface cannot photosynthesize and their population will decrease. This disrupts the base of the food chain.
Fortunately, there are a few herbicides available that can provide fairly good control of duckweed, especially when combined with methods to manage excessive nutrients and increasing wind access by removing trees along the edge of the pond. To find out more about control of these and other aquatic weeds, see HGIC 1714, Aquatic Weed Control Overview, HGIC 1715, Biological Control of Aquatic Weeds, and HGIC 1720, Chemical Control of Aquatic Weeds.