Homeowners typically find duckweed infestations in ponds near woods or surrounded by plants, fuelled by the nutrient-rich waters and the lack of wind. The free-floating duckweed quickly covers the entire surface of the pond and blocks sunlight from reaching submerged plants, hampering the photosynthesis process. The lack of photosynthesis results in a critical shortage of oxygen in the pond, which can very quickly kill plants and fish. Early identification and removal is key to combating duckweed.
Wait for a windy day, if possible. Strong winds push the duckweed to one side of the pond, making it much easier to collect.
Skim the surface of the pond and collect the duckweed, using a long-handled, fine mesh skimming net, found at most pool supply stores. Dump the duckweed into a large container, such as a bucket or trash bag. Pay close attention around plants or the edge of the pond, where duckweed may have pooled and collected. Continue until all the reachable duckweed has been harvested and removed.
Don a pair of hip waders and travel into the pond to reach duckweed further in the water. Leave the container used to hold the harvested duckweed next to the shore line. Secure the container in place by setting it in a hole or propping it up with other objects to prevent the harvested duckweed from spilling back into the pond. Continue skimming and removing duckweed until all traces of it are removed from the surface.
Carry the container far away from the pond or any other bodies of water. Empty the duckweed into trash bags. Tie the trash bags securely and put them in a lidded dustbin. Clean the inside of the container with hot water if you plan on using it again.
Watch the surface of the pond for any signs of a returning infestation. Immediately skim any new duckweed from the pond. Continue monitoring the pond until the duckweed doesn't return.
Stocking your pond with koi can help keep duckweed controlled, once large infestations have been physically removed. Combating duckweed manually is cheap but difficult. Consider spending the money on chemical controls if you find the duckweed returning repeatedly. If possible, remove any wind-blocking plants or trees away from the pond. Duckweed thrives in stagnant water with lots of nutrients. Moving plants away from the pond takes away the nutrients duckweed requires and opens the pond to stronger winds that blow duckweed onto the land, where it dies.