Lesser Duckweed is a native perennial floating aquatic plant that forms colonies. The plant body is not differentiated into stem and leaves. It floats at or near the surface of the water.
Leaves: What could be called a leaf has 3 faint nerves. The plant is frequently found with several (up to 5 or more) together as a small colony. As more of these leaves grow, they separate to become separate plants, which is the main means of propagation. The shape of the body is flattened, a bit egg shaped, 1.3 to 2x as long as wide, and often inflated on one side. These bodies are called fronds (as in ferns). Each frond has a single root dangling from the underside.
Flowers: Tiny (microscopic) flowers occur from time to time at the edge of a frond or on the upper surface. They have 2 stamens but lack petals and sepals.
Fruit: Mature flowers form a bladder like receptacle containing the tiny seeds, or buds. When water temperatures drop toward the freezing mark, starches fill the body and the weight causes them to drop to the bottom where they overwinter. Warming water in the spring of the year dissipates the starches and they re-float to the surface.
Habitat: Duckweed is found in quite waters, forming large floating mats as the division of the body can occur rapidly. The large mats shade out light from plants beneath, giving Duckweed an area free of competition. This is an important food for ducks, fish and birds.
Names: The genus, Lemna is Greek referring to a lake or other body of water and the species name is used to refer to smaller of more minor plants. Some authorities, such as U of M Herbarium, have now placed the Lemnas in the family Araceae (Arum). Flora of North America has not yet reflected this in their digital file and still lists the family as Lemnaceae. The author name for the plant classification - 'L.' - refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: There is a Greater Duckweed, Spirodela polyrrhiza, which is widespread in Minnesota, but there the bodies have 5 to 15 roots per frond. However the most important distinction is that the University of Minnesota and the MN DNR report that most species identified as L. minor are really L. turionifera, the Turion Duckweed, and that that species is the widespread species, whereas L. minor is found infrequently. The differences are that L. turionifera has a more rounded body, 1 to 1.5x as long as wide, there is a row of tiny projections along the mid-line of the frond upper surface and usually some reddish color beneath. For overwintering, it forms small buds (turions) which drop to the bottom for overwintering and these form new plants in the spring.
Above: Duckweed forms large mats of vegetation, blocking light from lower growing plants. The plant bodies grow together and then new growth splits off to form a new plant.
Below: A distinguishing characteristic of the Lemna genus is the single root from each plant body.
Below: Development of Duckweed fronds in early spring.
Notes: Duckweed is indigenous to the Garden. Eloise Butler catalogued it on May 31, 1907. Lesser Duckweed is found throughout North America with only a few Canadian Provinces not reporting it and only South Carolina in the U.S. not reporting it. Within Minnesota, there are only a dozen counties not reporting the plant, almost all in the Southern part of the state. However, Minnesota experts believe that most findings have been mis-classified. See 'comparisons' above.
Uses: Duckweed in the natural environment has beneficial aspects: By creating a mat on top of the water it keeps algae and mosquito breeding under control while provide food for ducks, shelter for tadpoles, shade beneath to help cool the water, plus it cleans the water. This last aspect is why duckweed is now used at many water treatment plants - the little plants help purify the water.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"