by Charles L. Mohler and Kimberly A. Stoner
Intercropping is an all-encompassing term for the practice of growing two or more crops in close proximity: in the same row or bed, or in rows or strips that are close enough for biological interaction. Mixed cropping, companion planting, relay cropping, interseeding, overseeding, underseeding, smother cropping, planting polycultures, and using living mulch are all forms of intercropping (see glossary at the end of this chapter). Intercropping includes the growing of two or more cash crops together. It also includes the growing of a cash crop with a cover crop or other non-cash crop that provides benefits to the primary crop or to the overall farm system. Cover crops can also be intercropped with one another. The purpose of this chapter is to outline some of the basic principles for using intercropping successfully and to relate these to the principles of crop rotation detailed in the rest of this manual.
“[Intercropping systems] call for careful timing of field operations, and they may necessitate special interventions to keep competition between the intercropped species in balance.”
Advantages of intercropping fall into three basic categories. First, an intercrop may use resources of light, water, and nutrients more efficiently than single crops planted in separate areas, and this can improve yields and income. Second, crop mixtures frequently have lower pest densities, especially of insect pests. This occurs both because the mixture confuses the insects and, if the mixture is chosen carefully, because the mixture attracts beneficial predators. Finally, intercropping may allow more effective management of cover crops.
The advantages of intercropping, however, do not come for free. Intercropping systems require additional management. They often call for careful timing of field operations, and they may necessitate special interventions to keep competition between the intercropped species in balance. A crop mix that works well in one year may fail the next if weather favors one crop over another. A mixture of crops with different growth forms or timing of development may make cultivation and use of mulches more difficult and less effective. Planting crops in alternate rows or strips greatly simplifies management and captures some of the benefits of intercropping for pest control. It may do little, however, to increase resource capture by the crops, unless alternating strips are close together.
Intercropping also poses a special problem for crop rotation. One fundamental principle of crop rotation is the separation of plant families in time. As explained in chapters 2 and 3, this is critical for management of diseases and, to a lesser extent, insects. If plants from two families are mixed in the same bed or field, however, achieving a substantial time lag before replanting either of those families may be difficult. Suppose, for example, that a farm grows an acre each of tomato, squash, broccoli, and mid-season lettuce. A simple rotation would put each of the crops in a different year, with a three-year interval before a crop is repeated on the same bed. If, however, the lettuce and tomato are grown together (see the next section), crops would be separated by only a two-year interval, which may be insufficient to keep some diseases under control (see Appendix 3). Thus, intercropping requires extra care and effort in planning and maintaining a viable crop rotation.
For an intercropping scheme to be useful, it should improve the overall economics of the farm. A new intercropping idea should be tested first on a relatively small area. This will allow evaluation of whether it fits into the overall management system and whether benefits outweigh extra costs, labor, or yield reduction. Note that some consequences of intercropping—such as better or worse weed control, or difficulties in timing planting or harvest—may not show up in a single test year.