The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.
Complete control is impossible and growth should be controlled before it reaches nuisance levels. Try the following for control and prevention of duckweed:
- On small ponds repeated raking or netting will keep the weed under control. Continuous removal is usually necessary
- On larger pools use a floating boom to sweep from end to end. Sweep at intervals from early in the season and continue until winter dormancy
- You can compost the removed weed
- Fit stop-boards at any upstream inlets to prevent duckweeds entering ponds or lakes
- Weed-eating water birds, such as domestic and ornamental ducks, moorhens and coots will provide some degree of control
- Grass carp will eat Lemna species
- Shading can reduce duckweed growth. This can be achieved by planting on the south side of the pond. Waterlilies and other plants with floating leaves can also substantially reduce the level of duckweed. The use of a fountain to disturb the surface may also help
No aquatic weedkillers are available to the amateur. Some herbicides can be used over waterways, but there are strict guidelines to follow and such work needs to be done by suitably trained professionals. The Environment Agency can advise further. The National Association of Agricultural Contractors (NAAC) has members who undertake such work. Proprietary products are available from suppliers of aquatic sundries.
Eco Pond have launched a product called Duckweed Control that is a non-chemical treatment. According to the manufacturers, it contains a bacterial culture that removes nutrients from the water to discourage duckweed.