Mother Earth News, Sep/Oct 78
Commercial growers maintain the genetic purity of their strains of seeds in several ways. One of the most common is to plant mutually fertile crops well apart from one another. The precise distance depends mainly on whether the plants are pollinated primarily by insects or the wind. Insect-pollinated vegetables require a minimum separation of 1/4 mile. Wind-pollinated types should be at least 1 to 1-1/2 miles apart.
Few backyard gardeners and small farmers can isolate their crops in this fashion of course ... so they must resort to other professional techniques. The most practical of those "other" methods for use on a small scale are:
- alternate planting
- hand pollinating