Great Duckweed
Spirodela polyrhiza
Duckweed family (Lemnaceae)

Description: This floating aquatic plant consists of a single thallus (a body that combines the functions of leaf and stem). This thallus is 3-9 mm. long and 2.5-7 mm. across; it is oval, broadly obovate, or orbicular in shape and its outer margin is smooth. The texture of the thallus is slightly succulent and it is filled with minute pockets of air, enabling it to float. The upper thallus surface is light to medium green, while the lower thallus surface is usually purplish red (rarely light green); both surfaces are glabrous and nearly flat. Toward one side of the upper surface of each thallus, there is a single node that is often red. About 5-12 veins originate from this node, curving inward (may require at least 10x magnification to see).

On rare occasions, tiny unisexual flowers are produced from 2 lateral pouches on the thallus. One pouch produces a single female (pistillate) flower, while the other pouch produces two male (staminate) flowers. A female flower consists of a single pistil, while a male flower consists of a single stamen; they have neither petals nor sepals. Both types of flowers are surrounded by tiny membranous spathes within their respective pouches. The blooming period can occur from early summer to early fall. The female flower is replaced by a tiny fruit (utricle) that is 1-1.5 mm. in length. This fruit contains 1-2 ribbed seeds. Instead of flowers, the lateral pouches more often produce vegetative buds that are connected to the mother plant by short slender stipes that soon wither away. A mother plant and its vegetative offspring may form temporary clusters of 2-5 thalli, but they eventually separate into individual plants. The root system of each plant consists of 3-15 rootlets that originate from a node on the underside of its thallus. These slender rootlets are up to 15 mm. long and their tips are pointed. When the weather becomes cooler during the autumn, a winter turion (a starchy bud-like offset) is produced on the underside of the thallus near the node of the rootlets. This turion is smaller in size than the thallus and either olive green or brown. The turion breaks free from the mother plant and sinks to the bottom of the water body, where it remains throughout the winter. During the warmer weather of spring, it rises to the surface of the water to begin the growth process again.

Cultivation: The preference is full or partial sun on fresh water that is mildly acidic to alkaline. The water should contain some nitrogen and other nutrients. Great Duckweed thrives in locations with stagnant or slow-moving water that receive some protection from wind and waves. Sometimes it is cultivated indoors for aquatic terrariums.

Range & Habitat: The native Great Duckweed is occasional to fairly common throughout Illinois. It has a world-wide distribution, occurring in North America, South America, Eurasia, and other parts of the world. Habitats include quiet inlets of lakes, ponds, backwaters of rivers, creeks with slow-moving currents, and marshes. This aquatic plant is found in both sandy and non-sandy wetlands.

Faunal Associations: Many of the insects that feed on Lemna spp. probably also feed on Great Duckweed. Some turtles have been observed to feed on this plant, specifically: Chrysemys picta (Painted Turtle), Emys blandingii (Blanding's Turtle), Sternotherus odoratus (Musk Turtle), and Trachemys scripta (Slider). Similarly, the American Coot, Sora Rail, and several species of ducks also feed on this species, as do carp and probably other omnivorous fishes. Dense mats of Great Duckweed and their abundant rootlets provide habitat for tiny aquatic organisms of various kinds. Because the rootlets are somewhat sticky when they are wet, such animals as muskrats, beavers, and probably some wetland birds transport this plant on their fur or feathers from one wetland to another.

Photographic Location: The plants were observed at a sandy marsh of the Heron Boardwalk in Vermilion County, Illinois. The photographs were taken indoors.

Among plants in the Lemnaceae (Duckweed family), Great Duckweed is one of the easiest to identify because of the large size of its thallus and its abundant rootlets. In contrast, Lemna spp. (Duckweed) have only one rootlet per thallus, while Wolffia spp. (Watermeal) have no rootlets. Other distinctive characteristics of Great Duckweed include its red upper node, abundant veins of its thallus (5 or more), and the purplish red underside of its thallus. An introduced species in Illinois, Landoltia punctata (Asiatic Duckweed), can be distinguished by the smaller size and more narrow shape of its thalli; it also has fewer rootlets per thallus (typically 2-5). On the basis of genetic and other evidence, some authors have assigned species in the Lemnaceae to another family of plants (Araceae). Other common names of Spirodela polyrhiza are Giant Duckweed and Big Duckweed.