For Pennsylvania pond and lakefront owners. Every fact carries the date we checked it.
Pond & Shore PAlakepennsylvania.us
Menu
Check my project

Plant guide · Floating plants

Watermeal (Wolffia species)

The world's smallest flowering plants: pinhead-sized green specks with no roots that can coat a pond like floating cornmeal.

Pond surface covered with tiny granular green watermeal mixed with small duckweed fronds.
Columbian watermeal mixed with duckweed. Photo: Stefan.lefnaer, CC BY-SA 4.0 (source)

What it looks like

  • Very small oval specks with no roots, about the size of a pinhead.
  • Looks like grainy green cornmeal or grass seed floating on the water.
  • Usually grows mixed in with duckweed.

Easily confused with

  • Duckweed: larger fronds, each with a hanging root.
  • Cyanobacteria: specks can look similar, but watermeal is a firm green plant you can scoop up and feel.

Good and bad for your pond

  • Food for waterfowl and other animals.
  • Like duckweed, a full cover blocks light and lowers oxygen as it decays.

Native or invasive?

Native

Native. Columbian watermeal is native (USDA PLANTS); three watermeal species occur in Pennsylvania (Penn State).

USDA PLANTS: WOCO (lower 48 states: native)

Do grass carp eat it?

Grass carp eat it

PFBC lists watermeal among plants grass carp prefer, but Penn State says no biological control works well for watermeal. Don't count on grass carp for it.

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. Fewer ducks and geese means fewer new introductions.

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4, 2026 · Source: Filamentous Algae (updated May 19, 2025) and the companion Penn State plant fact sheets This may be outdated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

Skim or net it off (fine mesh) and dispose of it far from the water; aeration that disturbs the surface helps herd it to the edges.

Biological options

No reliable option (see above).

Chemical options (permit first)

Very few products are labeled for watermeal. 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