Remove duckweed from your pond by hand, if you have a small problem. Remove a few duckweeds at a time as they appear in your pond to prevent a large problem from developing. Remove the duckweed in the spring if it does not reproduce too quickly through the sporing process.
Kill duckweed with herbicides, such as diquat or 2,4-D. Mix the herbicides according to label instructions to create a solution that can be sprayed directly on the plants. Re-apply the solution as necessary to get the desired results. Remove as much duckweed as possible because it can change oxygen levels and keep sunlight from getting into your pond.
Put fish, such as koi or grass carp, or waterfowl, such as mallards, in the pond to try to control small amounts of duckweed. Add them into your pond during the spring to get better results.