Duckweed is also known as Lemna Minor, and can be found in freshwater ponds and moving streams. Duckweed can grow easily in ponds that have high nutrient levels and spread quickly. The weed is also called duckweed because it is often a food resource for ducks and other animals. However, most people see the floating, dark green weeds as an eyesore for their ponds. The duckweed is beneficial if your pond is full of weed, and many pond experts would recommend leaving at least some duckweed in the pond. Remove all of the duckweed if you don't have any fish or want wildlife in your pond.
Put a hose in your pond and over flood the pond with water if you have no fish in it. Let the duckweed flow out of the pond and then dispose of it all in a plastic bag. Make sure to wear latex gloves when handling the duckweed.
Use a pool cleaning net to scoop any excess duckweed that may be stuck to the sides of your pond. If you have fish in your pond and only a small amount of duckweed to remove you also just use a net to remove the duckweed.
Remove all fish and drain your pond entirely for severe cases of duckweed that need to be eradicated. Clean your pond out using disinfectant cleaner and wash out the bottom completely using a hose.
Refill your pond with water and check the alkalinity of your pond by using test strips which can be purchased at you local pool cleaning or lawn care store. Add crushed limestone to your pond if the alkalinity is below 100 mg.
Add fish to your pond that will feed on the duckweed. Koi are the best to add and will eliminate the duckweed problem. However, the duckweed shouldn't return as long as you keep your pond clean and healthy.