Ponds & Lakes Q & A
Q: Duckweed has taken over my pond! What do I do to eradicate it and prevent future outbreaks? – Sue in Michigan
A: Common duckweed, or Lemna minor, can take over a pond in no time. Growing in dense colonies in quiet, undisturbed water, these tiny free-floating plants, if left unchecked, will blanket a pond or lake over the course of several seasons, depleting the water body of oxygen, destroying fish populations, and killing submerged plants by blocking the sunlight.
Most often, these green invaders are transported to your pond on the feet of waterfowl, such as ducks, geese or even herons. The plants stick to their feet or feathers and can be carried for miles. Though water fowl and some fish eat duckweed, it typically reproduces faster than the animals can consume it.
When controlling duckweed, you can use a fast-acting aquatic herbicide, like PondWeed Defense, to knock down the plant population. The contact herbicide is designed to work best on mature aquatic weeds in a contained environment, and you will need to apply it to the duckweed multiple times for effective short-term control.
To spot-treat duckweed, mix 1.5 gallon of PondWeed Defense with at least 1.5 gallons of tap water and spray directly on the pond’s surface using the Airmax Pond Sprayer. One gallon will treat 5,000 square feet. To ensure safe oxygen levels, treat your pond in thirds, waiting 10 to 12 days between treatments.
For longer-term control, use WhiteCap, or Fluridone. It’s the least-expensive method of treating an entire pond, easy to apply, safe for aquatic life and lasts an entire season. The herbicide is absorbed by the leaves, roots and stems directly from the water, and it works by inhibiting the weed’s ability to produce carotene, a pigment that protects the plant’s chlorophyll. Without carotene, the sun quickly degrades the green chlorophyll and the weed dies.
To treat one surface acre of duckweed in a 4 to 6-foot deep pond, we recommend you mix in a tank sprayer 32 ounces or 1 quart of WhiteCap with enough water to fill the tank. Place the spray nozzle directly under the water and disperse evenly around the pond. It can be applied in early spring before the weed even appears, which means you can get ahead of it before it becomes a problem. Keep in mind that it needs to stay in your pond for up to 90 days, so it’s not recommended to use in ponds with heavy overflow or during times of heavy rain. Also, WhiteCap will be degraded by the sun so we suggest to add Nature’s Blue Pond Dye or Black DyeMond Pond Dye right after treatment to ensure the longevity of the application.
POND TALK: What kinds of invasive aquatic plants have taken root in your lake or pond? What did you do to control them?