Pond coated with duckweed. Photo: Diane Oleson, Penn State Extension
So what exactly is the pond scum? Well, it could be any number of things. Sometimes it is watermeal or duckweed or both.
Duckweed and watermeal are free-floating aquatic plants that are commonly encountered in Pennsylvania and found in nutrient-rich ponds with stagnant or little flow.
Duckweed can be recognized by its small, single or grouped, round- to elliptical- shaped floating frond or leaf about the size of a pencil eraser, with a very short root that hangs from the underside of the plant.
Watermeal is identified by its very small, oval single frond or leaf that has no roots and is about the size of a pinhead. It has the appearance of green grass seed floating on the surface of the pond.
Both duckweed and watermeal produce flowers. In fact, they are the smallest flowering plants known, though the flowers are rarely observed.
Duckweed and watermeal both have value to the pond ecosystem because they serve as food for numerous types of organisms. They are especially important as a food source for waterfowl.
While these individual plants are small, they can reproduce prolifically and quickly cover the entire pond surface when growing conditions are right. This type of an infestation can block sunlight, inhibit oxygen exchange, and reduce dissolved oxygen concentrations in the pond water when the plants die and decay.
Long-term control of any type of overabundant aquatic plants is best accomplished by reducing or redirecting nutrient sources from the pond. This can be done by reducing fertilizer applications near the pond, maintaining septic systems properly, redirecting nutrient-rich runoff away from the pond, and maintaining vegetative buffer strips around your pond.
Multiple methods of control are available for both duckweed and watermeal. Combining and using these methods is usually most effective. Physical control of duckweed and watermeal is usually accomplished by netting or raking the plants off of the pond and then disposing of plant material away from the pond so that wind or runoff cannot transport it back into the pond. Another physical control strategy is the use of aeration. Duckweed and watermeal prefer to grow in stagnant water so aeration will discourage them from covering the pond.
A secondary biological control that can prevent infestation or reinfestation is limiting or deterring waterfowl access to the pond. While it was mentioned earlier that waterfowl feed on these plants, they are also a major transporter of duckweed and watermeal into ponds. These tiny plants lodge between the feathers of ducks and geese and then are freed when the waterfowl lands and swims on other ponds.
Only a few chemical controls on the market are labeled for use and are effective for the treatment of watermeal and duckweed. When used appropriately following the label instructions they can be safe and effective management tools.
If you have questions about the best management options for duckweed or watermeal in your pond, seek advice from a reputable information source before making a management decision.