Problems With Duckweed in Ponds

If you've just set up a small garden pond or a large fishing spot, you may already have noticed a disaster in the making: duckweed. This small green plant can creep its way into ponds, infest them, and choke out all other life forms. Here is a look at common problems that duckweed causes.

Problems With Duckweed in Ponds


Duckweed is a small, free-floating plant that multiplies rapidly. It may spread from pond to pond on the feathers of ducks. According to the Ohio State Extension Service, duckweed has an explosive reproductive capacity that can cause a pond to be covered in a matter of weeks. A duckweed-choked pond can appear as if it's covered with bits of bright green leaf. A person looking at a duckweed-covered pond may feel that they have stumbled upon a bog or swamp.

Threat to Fish

According to the Ohio State Extension service, every summer, a few duckweed-covered ponds will block light from reaching other plants that live within the pond. When the plants that live in the pond cannot receive light, they cannot put oxygen back into the water. When the oxygen in the water is depleted, the fish gradually die out. Over time, fishermen may notice that the fishing quality of a duckweed-filled pond declines. This is a phenomenon known as "fish kill."


Duckweed is often considered a smaller pond problem, because unlike lakes, smaller ponds do not "turn over." Turning over occurs when the cooler water rises to the top of the lake. In larger lakes, the turnover typically happens as the seasons change. Turning over is a process that releases nutrients that are trapped in the bottom of the lake with decaying plant matter. These trapped nutrients are what duckweed thrives on.


While there are chemical means of controlling duckweed in ponds, many of these can be harsh and may kill other plant life in the pond. An alternative method is to skim duckweed out of the pond with fine-mesh screens and nets. This can be a time-consuming, labor-intensive process, and may prove futile, since duckweed multiplies so quickly. Instead, you can slow the infestation by aerating your pond and using mild chemicals to kill off duckweed.


Duckweed may be a symptom of other pond problems. Part of why duckweed is able to take root and grow in a pond is that the pond may already be out of balance, containing too much nitrogen. Duckweed thrives on this nitrogen-rich environment.