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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.
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.
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 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.
Complete control is impossible and growth should be controlled before it reaches nuisance levels. Try the following for control and prevention of duckweed:
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.
Pond algae and blanket weed
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