Much of agriculture today centers on annual plantings and harvestings of a single species over an extended area. Think of the wheat fields of Kansas with a single variety of wheat filling acre upon acre of the landscape. Perennials, as opposed to annuals, produce flowers and seeds more than once in their lifetime. In practical terms, perennials do not have to be planted annually. Perennial is a term usually applied to herbaceous plants or small shrubs rather than large shrubs or trees, but, in the strict sense used here, it applies to all plants that flower and produce
1 See Bjorn Lomborg, ed., Global Crises, Global Solutions, Cambridge University Press, Cambridge, 2004, for one listing of global problems.
seeds more than once. In some definitions, perennials are plants that last three or more seasons. For our purposes here, the longer, the better.
Polycultures are a bit harder to pin down. The classic wheat fields of Kansas represent a monoculture—the planting of a single species of plant on a given plot of land. Technically, a polyculture is one in which more than one plant species is planted on a plot of land in a given year. This opens up a variety of possibilities that can be divided into two main groups: sequential cropping and intercropping. Sequential cropping occurs when two or more varieties are planted on a given plot of land during the same year. This is polyculture that overlaps in space, but not in time. In some regions, sequential cropping can produce four crops a year.
Intercropping occurs when two or more varieties are growing at the same time on a given plot of land. This is polyculture that overlaps in time and space, which permits a wide variety of designs. In time, two or more cultures can be planted simultaneously or separately (sometimes called relay intercropping). In space, there are even more possibilities. Common spatial polycultures include row intercropping (where separate crops are in separate rows), strip intercropping (where crops are separated into wider strips of rows), and mixed intercropping (where two or more crops grow with no distinct row arrangement).
The number of cultures can vary as well. In Mexico, it is common to plant maize, beans, and squash on the same plot of land. Many backyard gardens contain a dozen or more "crops" on one plot of land. Polycultures in some parts of the world can easily reach 30 and more species on a given plot of land. Polycultures technically include even noncrop elements such as trees2 (which are perennials), and nonplant elements such as livestock and fish, but this is straying too far afield from our topic here.
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