Spirodela polyrrhiza (Greater Duckweed)
|Also known as:||Great Duckmeal, Common Duckmeat|
|Habitat:||part shade, sun; quiet, shallow water; lakes, ponds, pools, river bottoms, slow-moving streams, marshes|
|Bloom season:||July - September|
|Plant height:||less than 1 mm|
|Wetland Indicator Status:||GP: OBL MW: OBL NCNE: OBL|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Plants rarely flower. Flowers are contained in lateral pouches near the base of the plant, usually 1 flower per plant, occasionally 2. Flowers lack petals or sepals; male flowers have 2 stamens and females a single style.
Leaves and stems:
Leaves, known as fronds, float on the water's surface, are 2 to 8 mm (to ~1/3 inch) long, 1 to 1.5 times as long as wide, round or nearly so in outline, widest at or above the middle, mostly rounded at the tip, and lack a stem. The upper frond surface is bright to dark green, often with a red spot near the base from which 7 to 15+ palmate veins radiate, though the veins can be quite faint.
The lower surface is typically red to purple, sometimes partly green. Attached near the base of a frond is a group of 5 to 12+ roots that are up to about 1 inch long. A plant has a single frond; 2 to several are often connected together though may eventually separate.
Like the rare flower, fruit—a tiny, ribbed seed 1 to 1.5 mm long—is rarely produced. Reproduction is primarily vegetative, via offshoots known as turions, which are round to kidney-shaped, lack roots, and usually olive-brown in color. In the fall, turions drop off and sink to the bottom of the water where they overwinter, floating back up when temperatures warm in spring, then growing, developing roots, and starting the cycle over again.
Greater Duckweed is very common in Minnesota and is found in the quiet waters of lakes, ponds, river bottoms and slow-moving streams, frequently mixed with Watermeals (Wolffia) and other Duckweeds (Lemna). It is most easily confused with Lemna Duckweeds, which have smaller fronds (to 5 mm long) with only one root, where Spirodela has multiple roots per frond.
While Spirodela polyrrhiza is the only species of the genus currently in Minnesota, a similar non-native species, Landoltia punctata (a.k.a. Spirodela punctata) is considered invasive to our south and may already be in southern Wisconsin; it has proportionately narrower fronds, more elliptic in shape, typically 2 to 5 roots and veins per frond, has a central line of minute bumps on the upper frond surface and is usually green on the underside.
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- Greater Duckweed plants
- Greater Duckweed with Wolffia
- Greater Duckweed habitat
- comparison of Spirodela, Lemna and Wolffia
Photos by K. Chayka taken in Hennepin County. Photos by Peter M. Dziuk taken in Ramsey and Washington counties.
Have you seen this plant in Minnesota, or have any other comments about it?