Weeds

WEEDS

WEEDS. Etymologically, "weed" derives from the Old English word for "grass" or "herb," but during the Middle Ages the meaning has changed to indicate an undesirable plant that grows where it is not wanted, especially among agricultural plots. This has historically been the primary meaning of the word, although in the nineteenth century, American writers grew increasingly aware that calling a plant a "weed" was an arbitrary human judgment, as there is no natural category of weeds. In the words of Ralph Waldo Emerson, a weed "is a plant whose virtues have not yet been discovered." Today, biologists tend to share that opinion, since many of the plants that are designated as weeds are, in fact, closely related to popular crops. Indeed, "weed" has fallen out of usage among biologists, although those who study agriculture still find the term useful in discussions of weed control and management.

American weed control only developed out of the manual methods of pulling and hoeing in the early twentieth century, when salts and other chemicals began to be used as herbicides. However, since the 1970s, as environmental and health concerns have been raised, less toxic methods of weed control have been explored, although it has been found that any interference can have unintended ecological effects. For example, the introduction of a natural predator of an unwanted species—termed "biological control"—can devastate other local species or even, by reducing competition, cause a different species to grow out of control.

Moreover, "weed" has recently developed a new meaning in North America as a term that is applied to socalled invasive species, or non-native plants. Throughout the history of the Americas, as people have immigrated they have tended to bring along the flora and fauna of their homeland, thus intentionally—and at times unintentionally—introducing new species to the continents. Some of these non-native species have multiplied to such an extent that they threaten, or have already destroyed, the biological balance of local environments. This problem has been especially pronounced in Hawaii, Florida, California, and New York State. However, the term "weed" is generally not applied to all introduced or non-native plants but rather to those that are doing the greatest harm to biodiversity and are least controllable through human interference.

Scientists have discovered certain common characteristics among many of the most successful invasive species. They tend to be able to flourish in a variety of climactic zones and to reproduce easily and quickly over long periods with small seeds that are less likely to be eaten. However, non-native plants may also have an advantage in that they can exploit unfilled niches in their new lands while perhaps avoiding traditional enemies. Modern mobility and faster forms of transportation are exacerbating the problem in America and around the world.

Some of the most notorious invasive weeds in America today include kudzu, tumbleweeds, and leafy spurge. Kudzu, from Japan and perhaps originally China, is a semi-woody vine that came to dominate much of the American Southeast in the later twentieth century. Its introduction was encouraged by the American government early in the century to help improve soil and stop erosion, and attempts have continued for decades to undo the ecological damage that its widespread planting and subsequent spread have caused.

Tumbleweeds are now considered to be emblematic of the American West, and some tumbleweed species are indeed native to North America, while others originated in Europe and Asia. They do well with little water and were once cultivated in the hopes of being a food source for livestock. Leafy spurge, which was introduced from Europe and Asia in the early nineteenth century, is believed to be harmful to cattle if eaten. As with kudzu, attempts are being made to control tumbleweeds, leafy spurge, and other invasive weeds through biological, chemical, and manual methods to prevent further environmental and economic damage.

The history of American weeds is not only the story of importations and largely unsuccessful attempts to control non-native species, for native American species have also traveled to new lands. Notoriously, native ragweed, whose pollen causes Americans with hay fever to suffer every fall, has made an appearance in Europe, where it is spreading despite attempts to control it.

BIBLIOGRAPHY

Van Driesche, Jason, and Roy van Driesche. Nature Out of Place: Biological Invasions in the Global Age. Washington, D.C.: Island Press, 2000.

Zimdahl, Robert L. Fundamentals of Weed Science. San Diego, Calif.: Academic Press, 1993.

Caroline R. Sherman

See also Agriculture .