Common Duckweed

What is common duckweed?

Common duckweed is a native floating plant and one of the smallest flowering plants in the world. Each plant is just a tiny green frond with a single root, but together they can blanket a pond from bank to bank. Once you scoop some up, common duckweed identification is simple: look for smooth, oval fronds about the size of a pencil eraser, each trailing one root.

On its own duckweed is not a villain. It feeds waterfowl and fish, soaks up nutrients and helps keep algae down. It becomes a problem when too much nitrogen and phosphorus reach a still pond from fertilized lawns, crop fields, barnyards or failing septic systems. Then the mat shades out everything below, cuts oxygen exchange at the surface and, when it dies back, uses up dissolved oxygen as it rots, which can kill fish.

Common Duckweed identification: what to look for

Skim a cup of pond water and look at the floating bits up close. Count the roots under each frond and note the size; duckweed, watermeal and giant duckweed often grow mixed together, and telling them apart matters because they respond differently to control.

• Fronds: Flat, oval to rounded, bright green and smooth-edged, about 1/4 inch long, floating singly or a few joined together.
• Roots: A single slender root dangles below each frond. No root means watermeal; two or more means giant duckweed.
• Texture: Fleshy, smooth and hairless on top, with a plain, unlobed edge.
• Flowers: Rare and so small they are almost never noticed.
• Growth: New fronds bud from old ones, so a few plants can spread into a solid carpet on calm, fertile water.

Where it grows in the United States

Common duckweed is native across almost all of the United States, Alaska and Canada, and it grows from roughly USDA hardiness zone 4 to zone 10. You will find it from northern farm ponds to southern lakes wherever water sits still.

It does best in calm, nutrient-rich water with little flow, such as stock ponds, stormwater ponds, sheltered coves and ditches, and it can also grow on wet mud. Ponds downhill from lawns, fields, livestock areas or septic systems are prime candidates, because those nutrients feed it. Visiting ducks and geese often carry it in.

Common Duckweed look-alikes

A few plants get mixed up with it:

• Watermeal (Wolffia spp.): Far smaller, about the size of a pinhead or a floating grass seed, and it has no roots at all. There is no biological control for it.
• Giant duckweed (Spirodela spp.): Larger fronds with two or more roots each and usually a purple or red spot on the frond.

How to get rid of common duckweed

Pulling, digging and mowing: On a small pond, net or rake duckweed off the surface and pile it well away from the water so it cannot wash back in. Pumps and surface skimmers work well on small ponds and remove nutrients along with the plants. A fountain or aerator that keeps the surface moving pushes duckweed to the edges, where it is easy to net. Expect to repeat removal through the season unless you also cut the nutrients feeding it.

Cultural control matters most, because the right conditions keep it from coming back.

• Cut fertilizer use near the pond and keep a buffer strip of tall grass or other plants along the bank to catch runoff.
• Maintain septic systems and redirect barnyard or field runoff away from the pond.
• Add aeration or water movement; duckweed favors still water.
• Discourage resident ducks and geese, which spread it between ponds.
• Grass carp give only limited control, often eating other plants first, and require a state permit and an approved supplier.

Herbicides: Penn State Extension notes that fluridone generally controls duckweed better than diquat, but it is non-selective and works best in ponds with little or no outflow. Diquat often needs several treatments. Flumioxazin is a fast contact product that works best on young, actively growing plants when the water pH is below 8.5. Texas A&M AgriLife also rates carfentrazone and penoxsulam highly. Confirm the plant before you treat. Treat before the mat covers most of the pond, and treat heavy cover in sections about two weeks apart, because a big die-off can drop oxygen and kill fish. Use only aquatic-labeled products and follow the label exactly; the label is the law. Your state may require a permit to treat pond water, so check with your state environmental agency first. Without nutrient control, expect to treat every year. If you hire someone to spray, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Keeping it from coming back

Duckweed is a symptom of rich, still water. The long-term fix is keeping nutrients out: reduce lawn and field fertilizer near the pond, maintain buffer strips, fix septic problems and divert runoff. Keep some water movement with an aerator, and avoid moving plants, nets or equipment from a duckweed pond to a clean one.

Questions we get about common duckweed

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