Greater Duckweed (Spirodela polyrhiza) — identification, habitat & look-alikes

Greater duckweed is one of the largest duckweeds, yet each plant is still only about 1/8–3/8 inch (3–10 mm) across. Look underneath and you'll see a reddish-purple underside and a little bundle of roots, which sets it apart from the smaller one-rooted duckweeds it often floats with. It's native to North America and common on quiet ponds and marshes.

Appearance

Appearance: Each plant is a single flat, roundish green with no true stem or leaves. The underside is usually red to purple, and there's often a red spot near the base on top.
Growth form: A cluster of roughly 5–20 fine roots, about 1/4–1 inch (0.5–3 cm) long, hangs from each frond. Fronds bud off new ones and often float in groups of 2–5.
Flowers: Tiny flowers form in a pouch on the frond, but they are rarely produced.
Flowering time: July–September (rarely seen)

Look-alikes

Common duckweedLemna minor

Lemna duckweeds have smaller fronds (up to about 3/16 inch, 5 mm) with just one root each. Greater duckweed has a bundle of roots and usually a purple underside.

Why they get mixed upThey often grow mixed together on the same pond.

Dotted duckweedLandoltia punctata

Dotted duckweed has narrower, more oval fronds with only 2–5 roots and is usually green underneath.

Why they get mixed upIt was once placed in the same genus and also has more than one root.

WatermealWolffia

Watermeal plants are far smaller, like tiny green grains, and have no roots at all.

Why they get mixed upIt forms the same kind of green film on still water, often mixed in with greater duckweed.

Where it lives

Typical habitats: ponds
lakes
marshes
slow streams
Range: Found nearly worldwide in fresh water, including much of North America. In Minnesota, for example, it's very common.
Preferred conditions: Prefers calm, shallow water sheltered from wind, in sun or part shade. It can spread into thick mats on warm, nutrient-rich water.
Flowering: Flowers Jul–Sep
Flowers Jul–Sep

Life cycle

Reproduction: In fall it forms rootless, starch-filled buds called that sink to the bottom for winter. When the water warms to about 59°F (15°C), they float up and start new plants.

Growing it

Propagation: Spreads mostly by budding new fronds through spring and summer.

Conservation

IUCN Red List: LCLeast ConcernLow risk of extinction

Culture and stories

Culture and stories: Because it grows fast and has a small genome, scientists study it for cleaning nutrients and metals out of water and as a possible source of biofuel.