Duckweed is the common name for a group of very small aquatic plants. The plants are the smallest flowering plants on earth, according to Iowa State University. Duckweed spreads rapidly and is quite unattractive. It is also harmful to fish and other plants living below the surface of the water. Duckweed is widely found growing on still bodies of water throughout the United States.
There are three main types of duckweed. The small duckweeds are Lemna valdiviana and Wolffia. They are the most commonly found type of duckweed, according to Iowa State University. Small duckweeds are distinguishable for their pairs of floating, shoe-shaped leaves. Each pair of leaves has a single root dangling down into the water. Wolffia, however, are so small that they have no roots.
The second group are the giant duckweeds, or Spirodela polyrhiza. These have larger groups of connected leaves with as many as nine dangling roots. The leaves are red on the undersides and have red dots on the top surface.
The final group is dotted duckweed (Landoltia punctata). These are distinguishable by their pairs of leaves with red undersides, and two to four dangling roots. Unlike their name implies, they do not have dots on the tops of the leaves like the giant duckweeds do.
Climate and Habitat
Duckweed plants can be found growing in still, stagnant and boggy waters throughout the United States. They cannot tolerate moving water, but are otherwise hardy. In fact, these plants can cover the entire surface of a pond in as little as one growing season. Duckweed can be found in urban areas, such as in ponds in Central Park in New York City, and in rural areas. They grow best in warm, nutrient-rich waters, which means a large duckweed population can be indicative of a nutrient imbalance.
Duckweed can cause many problems and is even considered invasive in many states, including Ohio. It can crowd-out desirable water plants, such as lilies. When it covers the surface of the water, it blocks out the sunlight, stopping the process of photosynthesis and disturbing the balance of oxygen in the water. This lack of oxygen can even kill fish in severe cases. Finally, duckweed is unattractive. People do not usually want to swim in ponds and lakes that are covered with this plant.
Duckweed is hard to control once established, so the best thing to do is prevent it from growing in the first place. There are two main ways to prevent duckweed growth, according to Ohio State University. The first is to make sure your water is not nutrient-rich. Do not let fertilizers run into the water from nearby soil. Remove plant matter before it can decompose. Second, aerate the pond. This will circulate the waters of the pond, moving the bottom water up to the top, preventing nutrient build-up and setting the water in motion so that it becomes inhospitable to the duckweed.
The best way to remove established duckweed is to simply scoop it out with a fine-mesh net. This takes a lot of time and effort, and is usually only effective on small ponds. Many lakes and bogs are much too large for this to work. Carp and koi are fish that will eat duckweed, so the addition of these fish to a home pond can control the population of the plant. These fish cannot keep up with a large population of duckweed, so they should be added before the duckweed appears in the spring. Some herbicides can kill duckweed, but these may also damage desirable plants and fish.