Weeds are the most costly category of agricultural pests. Worldwide, weeds cause more yield loss and add more to farmers’ production costs than insect pests, crop pathogens, root-feeding nematodes, or warm-blooded pests (rodents, birds, deer, and other large grazers). Because organic farming principles and standards preclude the use of most herbicides, many organic farmers consider weeds their most serious barrier to successful organic production, and effective organic weed control a top research priority. In particular, weeds are a constant fact of life in vegetable crops. With a little diligence and timely weeding, the home gardener can turn most weeds into beneficial organic matter. However, weed control costs can really add up in a one-acre market garden, and a weedy vegetable field at the 10–100-acre scale can spell a crop failure. Having an ecological understanding of weeds is the foundation of an effective organic weed management program that can make the difference between success and failure.
If it weren’t for weeds, the world would have lost more topsoil than it has to date, and humankind might have suffered mass starvation by now. Why? Because the plants we call weeds do a vital job in ecosystems: they quickly establish in, protect, and restore soil that has been left exposed by natural and human-caused disturbances. Watch any logged-over area in the Appalachian region of the eastern United States, and you will see precious topsoil washing away in each heavy rain—until the brambles (Rubus spp.), greenbrier (Smilax spp.), pokeweed (Phytolacca americana), poison ivy (Toxicodendron radicans), and other brushy weeds cover the ground with their impenetrable tangle. Other regions have their own characteristic guilds of pioneer plants that start the restoration process after timber harvest, natural disaster, or other disturbance has stripped the soil of vegetation. These pioneer plants initiate the process of ecological succession that, if left uninterrupted, will eventually restore the climax plant community native to the region: forest, savanna, prairie, chaparral, and so forth.
In agriculture and horticulture, humans replace the native climax vegetation with a suite of domesticated plant species chosen for their value as food, fodder, fiber, and fuel. Most agricultural systems severely reduce the diversity of the plant community and impose some form of repeated disturbance designed to maintain conditions favorable to growth of the chosen crop species. This disturbance inevitably elicits a "weed response" from nature, especially in annual cropping systems such as vegetables, in which the soil is frequently tilled or otherwise prepared for planting. Successful organic weed control—managing the land’s natural weed response to cultivation—begins with an ecological understanding of weeds and their roles in the farm or garden ecosystem (Altieri, 1995; Sullivan, 2003).
