How to Identify and Control Duckweed

Sept 30, 2013 – Duckweed is commonly mistaken for algae and is associated with many water quality problems in ponds and stagnant water bodies. Duckweed will remove plant nutrients from water, block sunlight and compete with natural algae growth.

While duckweed is an indicator that excessive nutrients exist in the water, it doesn’t really cause water quality problems. It can actually improve water quality by removing elements like phosphorus and nitrogen from the water while naturally filtering out unwanted materials.

With the right conditions such as nutrients, sunlight, and shelter from wind, duckweed can grow rampant by consuming phosphorus and algae (phytoplankton), which are lower on the pond-cycle food chain. Migrating birds as well as floods can even spread duckweed.

Duckweed growth is more desirable than algae since algae pose more problems for water use. Some algae can produce liver or nervous system toxins such as cyanobacteria or blue-green algae; and almost all algae will cause odor problems in water.

Duckweed is a small round or oval shaped plant that floats on the water surface. The smallest species is 2 millimeters or less in diameter, and the largest species is about 20 millimeters (or almost 1 inch) in diameter, which looks like little floating leaves on the surface.

Duckweed won’t normally thrive on sites exposed to wind or where flowing water occurs, and although it is often considered ugly, duckweed blooms can cover an entire water body with a green bio-mat containing millions of the small plants. If duckweed is covering a pond, be sure to check the oxygen levels in the water.

To test for low oxygen levels, follow this simple test:

  • Scoop some of the duckweed away from the surface and collect a water sample using a container such as a mason jar.
  • Look for zooplankton swimming in the water. Most healthy water bodies will have an active zooplankton atmosphere.
  • If the zooplankton are slightly red due to hemoglobin, the oxygen levels are too low and supplemental water aeration systems are required.

Managing Your Pond

It’s recommended that all ponds (whether or not they have duckweed) be continuously aerated year-round using good water airstone diffusion systems.

You should also remove any dead and decaying plant matter each fall, including the duckweed plants, before water temperatures get too cold.

Duckweed can be harvested by dragging a wooden board over the water surface towards the shoreline. Once close to the shore, remove the plants from the watershed to prevent them from rotting in the water.

Duckweed doesn’t only have water quality control benefits for ponds. Duckweed has also been used successfully to maintain oxygen levels. Some studies suggest that full coverage of duckweed will reduce water loss from evaporation on your pond by as much as 33%.

Duckweed also grows in stagnant, polluted waters, which makes this plant ideal for water reclamation areas as well.