Planting a wide variety of plants is another way to foil pests and diseases by encouraging diversity. Conventional lawns, a type of monoculture, are a good example. Planting a mixture of lawn grasses (especially disease-resistant ones) will help prevent diseases from sweeping through your lawn, because some plants will be susceptible and others won't. Replacing some of your lawn areas with a variety of tough groundcovers will also increase the overall diversity of the home landscape. It's an excellent alternative for sites on which lawn grasses won't naturally thrive.
The importance of using diversity is especially apparent if you look at long-lived plants such as trees and shrubs. Using a large planting of a single species of tree or shrub to screen a busy roadway, or planting the same shade tree that all your neighbors have, are two more examples of monocultures. A devastating pest or disease that appears on the scene can easily move from plant to plant and damage or wipe out the entire planting in a single season. Dutch elm disease, which decimated American elm plantings along thou sands of American streets in the mid-1900s, is a perfect example of the dangers of mass planting single species. If you've planted a variety of trees and shrubs for screening or shade, you have built-in protection.
Companion planting is another way to use diversity—and avoid monocultures— to foil pests. Mixing marigolds or strong-smelling herbs in the vegetable garden, as opposed to planting solid blocks or rows of a single crop, is effective, because many pests locate crops by smell. Interplanting can "hide" a crop by masking the odor that attracts pests. Some companion plants work by repelling pests. For example, basil is said to repel tomato horn-worms. For more on the many ways companion planting can be used to control pests and diseases, see page 419.
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