Duckweeds (Lemnacaeae family) are the smallest flowering plants in the world, according to Iowa State University. These tiny, floating plants are found all over the United States and are especially fond of still or stagnant waters. Duckweed is considered invasive and undesirable in many areas, as the green plants can completely cover the surface of a pond or lake, cutting off the sunlight and using up a large amount of the oxygen in the water.
Small Duckweed (Lemna valdiviana and Wolffia)
Small, or lemna, duckweed is the most common type of duckweed, according to Iowa State University. The fronds (leaves) of this plant, which are shaped like shoes, float on the surface of the water. Each plant features two or more fronds, which are joined at the tips. Each frond averages only 4 mm wide at the most.One root dangles from the group of leaves into the water. Duckweed plants do not have stems. Wolffia duckweed is the tiniest of the small duckweed plants, averaging only 1 mm wide. These do not even have roots, but simply float on the water, looking much like floating tiny green seeds.
Giant Duckweed (Spirodela polyrhiza)
Giant duckweed is larger than the Lemna species. The half-inch fronds or leaves are in bigger clumps (up to nine, according to the University of Florida). Each clump can have several roots dangling from the leaves instead of just one. Another major difference between the two is that giant duckweed plants are colored red on the undersides of the leaves. Each frond also has a dot on its upper surface.
Dotted Duckweed (Landoltia punctata)
This duckweed does not feature red dots like giant duckweeds do, which makes its common name somewhat confusing. They do have ridges on the leaves, however, which is where the name might have come from, according to the University of Florida. Dotted duckweed has groups of two fronds on average, which are shoe-shaped, and two to four roots dangling from the leaves. The leaves sometimes have red undersides, like giant duckweed.