Identifying Duckweed Species
Most people encounter duckweed as a green carpet on the water surface and assume they are dealing with a single species. In reality, what appears to be a uniform green covering may contain several Lemnaceae species growing together, each with different biology and slightly different management responses. For most practical management purposes, species-level identification is not necessary — all duckweed species respond to similar nutrient reduction and management strategies. However, identification is important when: conducting ecological assessments, reporting occurrences, determining whether watermeal (Wolffia) is present (which is more difficult to control than Lemna), or separating duckweed from filamentous algae or other floating material.
The Critical First Step: Is It Duckweed or Algae?
The most common identification confusion is between duckweed and various forms of algae or cyanobacteria that also form green surface mats. Key distinction: duckweed fronds are discrete, solid, plant-like structures — they have defined shape (oval, round, star-shaped) and can be picked up individually with your fingernail or tweezers. Algae and cyanobacteria form filamentous threads, amorphous mats, or powdery surface scums with no defined individual structure. If the green material has identifiable frond shapes — even very tiny ones — it is duckweed. If it is slimy, thread-like, or formless, it is algae or cyanobacteria. See the complete duckweed vs. algae comparison for more detail.
Common Duckweed (Lemna minor)
The most widespread and abundant duckweed in North America. Frond characteristics: oval to nearly circular, 1.5–5 mm long (roughly the size of a match head to an apple seed); upper surface flat or slightly domed, medium to light green; underside paler or same color; surface smooth; one root per frond (visible as a single white or pale thread hanging below the frond when held up to light); fronds often connected in groups of 2–3 before separating. Lemna minor floats flat on the surface in calm conditions, creating a smooth-looking mat. This is the species typically depicted in photos labeled "duckweed" and is the most likely species if you have a green surface carpet in a temperate North American pond or lake.
Giant Duckweed (Spirodela polyrhiza)
Noticeably larger than common duckweed: fronds 5–10 mm across (roughly the size of a small pea or lentil). Upper surface dark green; underside distinctively reddish-purple (a reliable identification feature — flip a frond and if it is purple below, it is Spirodela). Multiple roots per frond (7–21 roots), visible as a tuft of fine threads below the frond. Fronds are flattened and rounded. Often grows mixed with Lemna minor and is larger, which makes it easy to separate by size when both are present.
Dotted Duckweed (Landoltia punctata)
Recently reclassified from Spirodela. Fronds 2–5 mm, oval to elliptic. Underside purple-tinged with distinct reddish or purple dots visible under a hand lens (the "dotted" character of the name). Has 2–7 roots per frond, intermediate between Lemna and Spirodela. Common in the southeastern United States and California. Often mixed with other duckweed species. Distinguishing from Spirodela requires counting roots and examining the underside dots under magnification.
Watermeal (Wolffia spp.)
Watermeal species are the world's smallest plants, and identifying them as such is the key management information — they are distinct from all other duckweeds in having no roots. Fronds are tiny (0.3–1.5 mm), round to elliptic or slightly asymmetric, pale green, and rootless. They feel granular when rubbed between your fingers, like very fine sand or corn meal — hence the name "watermeal." Under a hand lens, they appear as smooth green spheroids with no appendages. Watermeal is often present in mixed duckweed populations and contributes to the difficulty of management — it is a finer particle than Lemna and is harder to remove by skimming or netting. When you pick up a scoop of what appears to be duckweed and it has a gritty, grainy texture, watermeal is almost certainly present.
Star Duckweed (Lemna trisulca)
Star duckweed has a unique growth form that sets it apart from all other duckweeds — fronds are narrow, elongate, and translucent, connected in star-shaped or Y-shaped colonies that typically grow submerged or semi-submerged just below the surface rather than floating flat. Individual fronds are 5–15 mm long and only 1–2 mm wide. This species rarely forms surface mats and is typically not a management concern. Its translucent appearance and submerged growth make it distinctive. It is most often found in clearer water bodies than other duckweed species.
Collection and Examination Tips
To identify duckweed species: collect a small sample in a container of water or on a white plate; allow the surface to settle and fronds to float naturally; examine with a 10x hand lens; count roots per frond; note upper and lower surface color; measure frond size against a ruler. For definitive identification, particularly of Wolffia species, a compound microscope (40–100x) is needed. Most management programs do not require species-level identification — presence of Lemnaceae is sufficient for management decisions. For management guidance, see duckweed control methods.
References
- Landolt, E. (1986). Biosystematic Investigations in the Family of Duckweeds (Lemnaceae). Geobotanical Institute ETH.
- Bog, M., et al. (2020). A key to determination of duckweeds. Plants 9(5):617.