Duckweed

Duckweed is a familiar sight to pond owners. The tiny, rounded leaves float on the water surface, resembling a mass of young cress plants. They multiply rapidly and quickly fill any open surface of water unless regularly cleared.

Duckweed on a pond. Credit:RHS/John Trenholm.

Quick facts

Common name Duckweed
Botanical name Lemna minuta, L. gibba and L. minor
Areas affected Ponds and water features
Main causes Floating weed
Timing Lemna minuta seen year round, other species seen late spring to autumn; treat when seen

What is duckweed?

Native and introduced duckweeds (Lemna minuta) can completely cover extensive areas of still or slow-moving water. They grow best in nutrient-rich waters and are common in garden ponds.

Appearance

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.

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

The problem

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.

Control

Non-chemical controls

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

Chemical controls

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.

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