Watermeal Identification & Treatment
Scientific Name: Wolffia spp. (W. columbiana, W. brasiliensis) Category: Floating Tier: 1 — High-Priority — Most prevalent and/or most damaging across the US
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Identification
Watermeal is the world's smallest flowering plant — individual plants are about the size of a grain of sand or pinhead (less than 1 mm). They are bright green, oval, and rootless, forming a granular-looking coating on the water surface that is often mistaken for algae or pollen. When you rub watermeal between your fingers, it feels gritty (unlike algae, which feels slimy). Key distinguishing features:
- Smallest aquatic plant — individual plants barely visible to the naked eye
- No roots, no leaves — just a tiny green body (thallus)
- Gritty texture when rubbed between fingers (vs. slimy algae)
- Forms a dense, granular green coating on the water surface
Commonly confused with: Planktonic algae blooms (slimy texture, often blue-green), duckweed (larger, has visible root), pollen (yellowish, not green, seasonal)
Treatment
Watermeal is one of the most difficult floating plants to control because of its tiny size and rapid reproduction.
Recommended Natural Waterscapes Products:
- Propeller (flumioxazin) — The most effective herbicide for watermeal. Must be applied when water pH is below 8.5. Apply early in the season before full coverage develops. Propeller 1lb
- Duckweed Destroyer Package — Combination treatment that addresses both duckweed and watermeal. Duckweed Destroyer
- Harvester (diquat) — Less effective on watermeal than on duckweed due to watermeal's tiny size and waxy surface, but can help as part of a combined approach. Harvester
Non-chemical: Aeration and surface agitation are critical. Reducing nutrient inputs is essential for long-term management.
Full Profile
- Native Range: Native to the Americas
- US Distribution: Found across the eastern and central US. Most common in EPA ecoregions: Eastern Temperate Forests, Southeastern Plains, Central Corn Belt Plains, Interior Plateau. USDA Zones 4–10.
- Regulatory Status: Not regulated (native species).
- Habitat: Still, nutrient-rich freshwater. Often found mixed with duckweed. Prefers sheltered, warm, high-nutrient conditions.
- Reproduction: Exclusively vegetative budding. Doubles population every 1–2 days under optimal conditions — even faster than duckweed.
- Ecological Benefits: Like duckweed, serves as waterfowl food and nutrient absorption. However, dense watermeal coverage is generally more harmful than duckweed because it blocks light more completely and creates severe oxygen depletion.
- Seasonal Behavior: Active spring through fall. Produces overwintering buds that sink in autumn.
Sources: Texas A&M AquaPlant | USDA PLANTS Database
Related Species
- Duckweed (Lemna minor and related species (L. gibba, L. turionifera, Spirodela polyrhiza)) — Floating, Tier 1
- Water Hyacinth (Eichhornia crassipes) — Floating, Tier 1
- Water Lettuce (Pistia stratiotes) — Floating, Tier 2
- Giant Salvinia (Salvinia molesta) — Floating, Tier 2
- European Frog's-bit (Hydrocharis morsus-ranae) — Floating, Tier 3
- Mosquito Fern (Azolla) (Azolla spp.) — Floating, Tier 3
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