Cold weather kills pond duckweeds, until spring

Late fall brings many complaints of weeds in ponds, but cold temperatures kill most for the winter.

Duckweed covering a pond.
Photo by Beth Clawson, Michigan State University Extension, at the Kellogg Forest pond, Augusta, MI

The late fall pond often displays its full flush of plants. It is common for duckweed in a pond to be mistaken for an overgrowth of algae, but it is not, it is a tiny flowering plant called Duckweed (Lemna minor). This simple, floating aquatic plant starts growing in the spring and flowers and reproduces by summer. It grows very fast and reproduces asexually growing buds and cloning itself. If it has a quiet pond or wetland with little wave action to grow in, it will multiply exponentially until the first freeze of winter. Often by the start of autumn, your pond will look like the one in the photo taken in mid-October.

Duckweed is not harmful to your pond or any fish or animals living in the pond. It serves to shade and keep the water cooler during the hot summer days. Duckweed takes up a lot of nitrogen from the water and can help control nutrient loading problems, however, to some, this growth is unsightly or shades so much surface area that other plants in the pond fail to thrive. When this happens, a couple of actions can be taken to manage it.  

Mechanical control

  • Skim the plant off the surface with small mesh nets to reduce their numbers and keep it in check. This works great for smaller ponds or ponded areas of a lake.
  • Install an aerator or fountain creating water movement to keep the area around the turbulence clear of duckweed and restrict its growth rate.
  • Introduce native plants and animals to your natural ponds.

Chemical control

  • If you are seeking a chemical control, use an herbicide rated for use in water. Treatment should begin early in the season for best control. This is not a permanent solution and repeat applications may be required.

It is important to note that this is a native plant and not necessarily harmful if present in a pond. The use of pond dyes mostly control algae and will not control this floating plant. Alternately, you could just learn to love it for its season and wait until winter’s cold temperatures kill off this tiny, flowering plant. At the same time, look for and correct the source of the nitrogen that is feeding accelerated growth in your pond if that is a problem.

For more information about pond management, contact Beth Clawson, MSU Extension Educator. To learn more about landscaping with native plants contact Michigan State University Extension Natural Resources educators who are working across Michigan to provide water quality educational programming and assistance.

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