Free-floating duckweed (Lemna minor) can quickly cover the surface of a pond or backyard water feature, shading the water so algae can't invade. It provides food and cover to pond fish, such as koi, in slow-moving waters. Duckweed can also become highly invasive in frost-free climates, but it dies off each winter in colder regions. It rarely becomes a problem in small backyard ponds where its growth range is limited. It grows in both sun and shade in U.S. Department of Agriculture plant hardiness zones 4 through 10.
Place a submersible pump, pond aerator or a bubbler in the pond to provide some oxygenation and water flow. Place the pump or bubbler in a position that allows some water in the pond to remain still, because duckweed needs still or slow-moving water movement to thrive.
Place a pond heater in the water to maintain a water temperature above 70 degrees Fahrenheit in cooler climates. Duckweed can survive and grow at any temperature above freezing, but it grows more slowly at temperatures below 70 F.
Skim out the excess duckweed and dispose of it when it forms an overly thick mat on the water's surface. If the duckweed completely covers the pond surface, it can shade out desirable pond plants or deplete oxygen in the water if it dies off, which can kill both plants and fish.
Place a pot in the water that is a few inches taller than the pond's depth if fish are eating all the duckweed. Grow the duckweed in the pot. The fish can still nibble at any duckweed that escapes the pot, but the plants inside the pot are safe and provide an ongoing supply of replacement plants for the rest of the pond.
Fill a container with 2 to 3 inches of moist soil and spread the duckweed over the soil surface to overwinter it indoors in cold climates. Cover the plants with 2 inches of water. Keep the duckweed in a frost-free area and replenish the water as needed until replanting in spring after frost danger has passed. Skim the duckweed from the container and place it back in the pond in spring after the last frost.