You cannot kill duckweed safely in the pond. You have to physically remove it if you want an organic wildlife safe method of duckweed control.
We want plant growth across the surface of a pond but Duckweed - Lemna minor - keeps growing until it covers the entire pond and will starve the submerged water of light, stopping the activity of the oxygenating plants.
You cannot use any chemical controls to kill the duckweed as these are not selective and will harm any other plants in your pond.
Unfortunately to kill duckweed:
- Remove every last piece or it will spread back.
- Wash every plant carefully from an infected pond. Then remain vigilant to remove any plantlet as soon as you see it. Birds feet can bring more in from a nearby pond.
- Duckweed can drop seed to the base of the pond in Autumn to reappear in Spring.
Using a skimmer to control duckweed:
These are the 2 versions of electrical skimmer unit available.
- Attach the Oase unit to a spare outlet on an existing pump so can be made as powerful as your pump will allow. Also useful for 'sucking in' fallen leaves in Autumn so they do not fall to the bottom of the pond. It sucks and creating a vortex and the surface growth collects into the mesh collecting basket.
- The Velda unit comes with its own pump and float attached so there is no choice of pump size. But it is easier to fit. It will not have such a good 'pulling power' to bring duckweed or leaves to its collecting basket as the Oase unit.