One question I get asked by new customers is about duckweed. Duckweed (Lemna minuta) can completely cover extensive areas of your pond mainly if it is still or slow-moving water. Duckweed grow best in nutrient-rich waters. Duckweeds are small, free-floating aquatic perennials that combine to form what looks like a green carpet on the surface of the water of your pond. Each plant consists of a single, rounded, the leaf-like body usually does not exceeding ¼” (5mm) in diameter floating on the surface with a small slender root below, and are always green in colour.
Reproduction of duckweed is vegetative, two daughter plants budding off from the adult plant. The plant mass can double in size every few days in optimum conditions in the summer.
Duckweed can be introduced to garden pools unknowingly, brought in with newly-acquired water plants. If possible plants should be quarantined for a few weeks before introducing them into your pond. It may also be brought in on the feet of water birds. (when I do a service on and pond that has duckweed I have to thoroughly clean all equipment before next use)
How to Control
To get duckweed completely under control is nearly impossible, growth should be controlled before it reaches nuisance levels. You can try the following non chemical control for the control and prevention of duckweed:
On small ponds raking or netting repeatedly 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.
Shading can reduce duckweed growth. 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
There are Chemical controls available but these are not available to the public. There are non chemical treatments that’s contain a bacterial culture that removes nutrients from the water to discourage duckweed. But these can also have a detrimental effect on your fish or plants.
Unfortunately once you have duck Weed you will never get rid of it completely, though using the tips above will help to keep it down to a minimum.