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Plant guide · Floating plants

Duckweed (Lemna minor and related species)

Tiny floating plants that can carpet a pond in weeks. Each frond is about the size of a pencil eraser with one little root hanging below.

Also called: lesser duckweed

Close view of hundreds of small round duckweed fronds floating on dark water.
Lesser duckweed (Lemna minor) on a pond surface. Photo: Christian Fischer, CC BY-SA 3.0 (source)

What it looks like

  • Small, round to oval floating fronds (leaves), single or in small groups.
  • Each frond is roughly the diameter of a pencil eraser, with a single root hanging underneath.
  • Several kinds grow in Pennsylvania; they often grow mixed with watermeal.
  • From a distance a solid green surface can be mistaken for algae.

Easily confused with

  • Watermeal: even smaller, no roots, and grainy like floating grass seed or cornmeal.
  • Cyanobacteria scum: specks or paint-like film, no leaves.

Good and bad for your pond

  • Food for many animals, especially waterfowl.
  • A full cover blocks light, cuts oxygen exchange and lowers oxygen when the plants die and decay.
  • Ducks and geese carry it from pond to pond in their feathers.

Native or invasive?

Native

Native. Common duckweed is native (USDA PLANTS).

USDA PLANTS: LEMI3 (lower 48 states: native)

Do grass carp eat it?

Grass carp eat it

PFBC lists duckweed among grass carp foods, but Penn State says grass carp usually can't eat enough of it to keep it under control and will eat other submerged plants first.

Control options

Prevention first

Penn State Extension's advice for every nuisance plant and algae problem starts with nutrients: overgrowth is a symptom of excess phosphorus and nitrogen from sources such as barnyards, crop fields, septic systems, lawns and golf courses. Cutting fertilizer near the pond, maintaining septic systems, diverting nutrient-rich runoff and keeping a vegetated buffer strip are the long-term fixes; without them, control becomes a yearly chore. Discouraging ducks and geese helps prevent reinfestation.

Checked Oct 4, 2026 · Source: Filamentous Algae (updated May 19, 2025) and the companion Penn State plant fact sheets This may be outdated

Remove it by hand or machine

Net, rake or skim it off and dispose of it away from the pond, where wind and runoff can't carry it back. Surface-disturbing aerators push it to the edges, where it is easier to scoop.

Biological options

Grass carp give partial control at best; see above.

Chemical options (permit first)

Only a few products are labeled for duckweed. Any herbicide or algaecide in a pond needs a joint DEP/PFBC Chapter 91 permit first, at any amount. Only products registered with the U.S. Environmental Protection Agency and with the Pennsylvania Department of Agriculture, and labeled specifically for aquatic use, can be listed on the Chapter 91 permit application. Treat in sections, early in the season: decaying plants can strip oxygen and kill fish.

Checked Oct 4, 2026 · Sources: Water Quality Permitting for Pesticide Applications in Pennsylvania: FAQ (rev. Feb 21, 2025), FAQ #2, #3, #8, #10, #11, #28, #29; Permit Requirement for Use of an Aquatic Herbicide in Ponds (updated Sep 8, 2025); Pond Management (FAQ: controlling submerged vegetation) This may be outdated

Follow the product label; the label is the law. A Chapter 91 permit is required before any herbicide or algaecide goes in a pond (see fact). Consider a licensed applicator.

Sources

Checked Oct 4, 2026