Duckweed is a little plant that can cause big trouble in a pond. They are free-floating plants that look like small green ovals less than 1/4 inch wide. They may or may not have a single root hanging down from them.
Because of their size, duckweed are often transported to new waterways by birds when they become lodged between their feathers. Duckweed is also known as watermeal, since all three are members of the duckweed family, Lemnaceae.
Duckweed reproduce by budding. A new daughter bud begins by growing from the parent plant and eventually breaks off to go its own way. Duckweed does this very quickly. According to Carole Lembi, a botany professor at Purdue University, duckweed can reproduce itself every 24 hours if given enough nutrients. This means that a single duckweed can turn into 32,000, by the end of 14 days.
Although duckweed is most commonly found in still, nutrient-rich wetlands and ponds, it can grow almost anywhere. It tolerates both sunny and shady areas. It is also tolerant of pH levels between 4.5 and 7.5 and temperatures between 68 and 86 degrees Fahrenheit. It will even tolerate slightly salty water. The only thing it tends to avoid is moving water.
Because duckweed covers the surface of a pond, it prevents sunlight from reaching the plants below. Without sunlight, photosynthesis stops, and plants and algae within the pond die. Since oxygen is one of the byproducts of photosynthesis, the oxygen levels in the pond drop and eventually kill off the fish.
Duckweed can quickly turn the surface of a pond green. They will cling to skin, feathers and fur of anything that enters the pond, making swimming unpleasant. As it slowly kills off plants in the pond, the bottom of the pond becomes a blackish muck. Eventually, the pond becomes even more attractive as the fish begin to die.
Controlling duckweed is difficult and costly. Control requires manually removing the duckweed or using herbicides, since biological controls are not very effective. Herbicides will only be effective if applied early in the infestation, before duckweed covers the entire surface of the pond. Sometimes the only effective method of getting rid of duckweed is to drain the pond completely.