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I’ve seen ponds completely covered over in green and chances are good they’ve been infested with duckweed. Duckweed is a small, floating-leaf plant that’s about the diameter of a pencil eraser. It also has a hair-like root that extends down below the leaf. But don’t let its size fool you. Millions of those tiny leaves can take over the entire surface of a pond.
Brian Swistock is an extension water quality specialist at Penn State University. He says duckweed can be spread by waterfowl.
"Ducks and geese move it around, because it’s such a small plant it sticks to their legs and to their feathers and so they can move it around from pond to pond very easily. And then once it gets into a pond, it can reproduce very, very quickly," says Swistock. "It really loves ponds that have very stagnant water and lots of nitrogen and phosphorous for it to grow."
A pond completely covered in duckweed looks terrible. However, the main hazard is that if it grows too abundantly, it can cut off oxygen in the water leading to the demise of fish and other aquatic life.
Swistock says there are a couple of options for duckweed control.
"You can mechanically remove it and that does provide a lot of advantages because you’re mechanically removing nutrients from the water as well which might help to prevent further growth of it. But, it’s not easy. You have to come up with some sort of a seine, or a net, or something you can skim it off the surface," he says. "Beyond that, mostly chemical approaches and there’s a variety of herbicides that work reasonably well on it."
Make your pond less hospitable for duckweed by reducing nitrogen and phosphorous levels in the water with buffer strips and controlling animal access. Anything you can do to keep the surface of the pond moving, such as aeration or flowing water, will also reduce its growth.