Open pollination is pollination by natural means such as insects, wind, and birds. Seeds from two parents of the same open pollinated variety will produce plants of the same variety. In contrast, seeds from two parents of a commercial hybrid variety, will not produce plants that are the same as the parent variety and are sometimes sterile.
Open pollinated vegetables reproduce by either self pollination or cross pollination. Self pollination occurs between male and female flower parts of the same flower or separate flowers on the same plant. Beans, peas, tomatoes, and lettuce are self-pollinating. Cross pollination occurs between male and female flower parts of two plants. Pollen is carried by bees and other insects, wind, and birds from one plant to another. Squash, cucumbers, melons, beets, carrots, corn, broccoli, cabbage, and kale are examples of cross pollinated vegetables.
For thousands of years before commercial seed production, all vegetable seeds were open pollinated and seeds were saved by families each year for planting the following year. These were passed down within families and communities from generation to generation and these varieties that we still have today are called heirlooms. Although all heirloom seeds are open pollinated, not all open pollinated seeds are heirlooms and new varieties are still being developed today.
Home gardeners can save seed from open pollinated varieties, but some are much easier to save than others. Self pollinated varieties are the easiest because the male and female flower parts are from the same plant guaranteeing the seed will be of the same variety. When saving seeds of a cross pollinated variety, care must be taken that a flower isn't pollinated by a different variety of the same species. For example, an acorn squash and a zucchini are the same species and will cross, producing squash with characteristics of both. Crossing can be prevented using physical barriers, distance, or timing plantings so different varieties of the same species flower at different times.