Giant duckweed is a native floating plant in Florida. Though very small, it is the largest of the duckweeds. Spirodela polyrhiza is frequently found growing in rivers, ponds, lakes, and sloughs from the peninsula west to the central panhandle of Florida (Wunderlin, 2003). It occurs almost always (estimated probability 99%) under natural conditions in wetlands.
Giant duckweed has two to three rounded leaves, which are usually connected. Giant duckweeds usually have several roots (up to nine) hanging beneath each leaf. The underleaf surface of giant duckweed is dark red.
This plant is easily confused with the exotic plant, Landoltia punctata. Landoltia duckweed is smaller than Spirodela polyrhiza, is more shoe-shaped, does not have a red dot on top, usually has only up to 4 roots, and sometimes has a red margin on the underneath of the leaves. The native, Spirodela polyrhiza, giant duckweed, is larger, has rounder leaves, some have a red dot on the top, has up to 9 and more roots, and is dark red underneath the leaves.
Also see small duckweed, Lemna valdiviana