Native (Lemna minor and L. gibba) and introduced (L. minuta) duckweeds can completely cover extensive areas of still or slow-moving water. They grow best in nutrient-rich waters and are common in garden ponds. The caterpillars of the Small China-mark moth feed on Lemna, building floating cases from small pieces of the leaves. This page looks at options for gardeners when duckweeds are becoming a problem.

Duckweeds are small, free-floating aquatic perennials that combine to form a green ‘carpet’ on the surface of the water. Each plant consists of a single, rounded, leaf-like body usually not exceeding 0.5cm (¼in) in diameter floating on the surface with a slender root below.

Some species usually overwinter on the bottom of the pool, to surface again the following spring. Lemna minuta, however, remains green throughout the winter.

The main means of reproduction is vegetative, two daughter plants budding off from the adult plant. The plant mass can double in size every two or three days in optimum conditions over summer.

Duckweed can be introduced to garden pools unknowingly, brought in with newly-acquired water plants. It is, therefore, good practice to quarantine purchases for a few weeks before introducing them. Duckweed may also be brought in on the feet of water birds.

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. 

Cultural control

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

Weedkiller control

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.