Duckweed vs. watermeal vs. filamentous algae: surface coverage ID
All three appear as green coverage on a pond surface and all three indicate excess nutrients. The treatment strategy differs, and so does the difficulty of the underlying problem.
| Attribute | |||
|---|---|---|---|
| Type | Tiny floating flowering plant | Tiniest flowering plant; a Wolffia | Colonial single-celled algae |
| Visible size | Oval fronds 2–5 mm | Specks under 1 mm — looks like green grit | Stringy mats, can cover acres |
| Texture | Smooth, distinct individual leaves | Like sand or grass clippings on the water | Slimy, hair-like, wraps around fingers |
| Has roots | Yes — single fine root visible underneath | No | No |
| Doubling time | 2–4 days | 3–7 days | Variable; explodes in 5–7 days under right conditions |
| Best control | Skimming, fluridone, addressing nutrient source | Fluridone (sometimes), skimming with fine mesh | Aeration, buffer plantings, mechanical mat removal, copper sulfate (last resort) |
| Hardest to remove | Hard at peak coverage; possible to skim | Very hard — slips through most filtration | Persistent if nutrient source is unaddressed |
Frequently asked questions
Will copper sulfate kill duckweed?
No. Copper sulfate is an algaecide. Duckweed and watermeal are flowering plants and require an aquatic herbicide (fluridone, diquat) or physical removal.
Why does my pond have all three at once?
All three thrive on excess phosphorus. A pond with chronic nutrient loading from lawn fertilizer or septic seepage typically supports duckweed, watermeal, and algae blooms in a rotating cycle. The lasting fix is reducing the nutrient input.