Plant of the Week
Watershield (Brasenia schreberi)
By Shannan Sharp
Watershield is an aquatic, perennial herb with floating leaves that grows in ponds, lakes, and slow moving streams. It is widespread in North America, and is found in South and Central America, the West Indies, eastern Asia, Africa, and eastern Australia. It is a member of the Cabombaceae (watershield) family.
Watershield is easily recognized by its leaves. They are oval shaped, peltate (the leaf stem attaches to the leaf in the middle of the blade, like a mushroom stalk), with the undersides covered with thick, jelly-like slime. Leaf blades are small, ¾ to 2½ inches wide and 1 to 4½ inches long. The top of the leaves are green, while the undersides and leaf stems are reddish purple. All underwater parts of this plant are covered with slime. This thick coat apparently protects plants from drying out during drought and may also deter mammals from eating it.
The flowers of watershield are small and inconspicuous, typical of wind-pollinated plants. Flowers have three to four sepals about ¾ inches long that are green underneath and reddish purple to maroon on top. There are usually four petals that are slightly longer and thinner than the sepals. They are reddish purple to maroon in color. Flowers usually have 18-36 stamens and 4-18 pistils in each flower.
Although watershield flowers are not showy, they have an interesting biology. Flower buds develop underwater and are covered with slime. Flowers bloom over a two-day period. On the first day the bud emerges above the water. Sepals and petals open and bend downward. Although stamens and pistils are present in each flower, on the first day of blooming, only the pistils emerge. Stalks of the pistils lengthen and spread outward over the petals. At night, the flower stalk bends and the flowers submerge beneath the water. On the second day, flowers emerge from the water again, but with the pistils retracted. The stamen stalks are lengthened and the anthers open. In this way flowers are cross-pollinated (Osborn and Schneider). After blooming, the sepals and petals fold up and submerge. Fruit develops underwater enclosed in the petals and sepals. Fruits are leathery and club shaped with one or two seeds each.
Although watershield is native to North America, it can become extremely weedy. This plant can quickly take over shallow ponds and lakes. It sometimes becomes so dense that it slows boat travel. Watershield plants secrete a number of chemicals that kill or inhibit growth of a wide range of bacteria, algae, and other plants (Elakovich and Wooten). By this mechanism, watershield kills off competing vegetation.
Although watershield can be a nuisance, it does benefit wildlife and has been used by humans. Waterfowl eat this plant. Its floating leaves provide shelter for fish and other aquatic organisms. Native Americans reportedly ate its tuberous roots. Young leaves and stems not yet covered in slime are eaten as salad greens in parts of Japan.
For More Information
Elakovich, Stella D., and Jean W. Wooten. 1987. Journal of Chemical Ecology. An examination of the phytotoxicity of the watershield, Brasenia schreberi. Vol. 13 (9): 1935-1940.
Flora of North America Editorial Committee, editors (FNA). 1997. Volume 3, Magnoliophyta: Magnoliidae and Hamameliidae. Oxford University Press, New York, NY. Accessed through: Flora of North America Online.
Godfrey, Robert K., and J. W. Wooten. 1981. Aquatic and wetland plant of the southeastern United States, dicotyledons. University of Georgia Press, Athens, Georgia.
Osborne, Jeffrey M., and Edward L. Schneider. 1988. Morphological studies of the Nymphaeaceae Sensu Lato. XVI. The floral biology of Brasenia Schreberi. Annals of the Missouri Botanical Garden. Vol. 75 (3): 778-794.
Yatskievych, George. 2006. Steyermark’s Flora of Missouri Volume 2, Revised Edition. Missouri Botanical Garden Press, St. Louis. Missouri.