Different varieties need to be separated as they will cross-pollinate. For example
Offered by Toni.
Mother Earth News, Sep/Oct 78
The chief problem you - the home seed grower - will face in your propagation activities is maintaining the purity and vigor of your favorite varieties of fruits and vegetables from one generation to the next ... and that largely boils down to preventing cross-pollination among closely related plants. For example, the various cole crops - cabbage, kale, collards, broccoli, and the like - all derive from a common ancestor and therefore cross readily. So do beets and Swiss chard ... turnips, rutabagas, and radishes ... and different strains of cucumbers or squash or melons. Some vegetables even accept pollen from certain wild cousins. Carrots, for example, will cross with Queen Annes lace (the common wild carrot imported into this country from Europe).
The offspring of crosses usually exhibit some - but not all - of the characteristics of each parent, depending on the roll of the genetic dice. But, as a general rule, they are decidedly inferior with respect to one or more traits that we value in garden vegetables (but which nature doesn't give a hoot about). Further uncontrolled crosses may eventually dilute the family virtues to such an extent that the strain becomes useless for cultivation. Then - as my neighbors would say - "Yer seed has run out!" So it s back to the catalogs and garden centers for a new supply.
Organic Gardneing, Sept/Oct 1992
Save Some Seeds For Next Season
by Suzanne Ashworth
Remember, only seeds from open-pollinated (not hybrid) plants will produce the same crop next year. And except for tomatoes, you need to be fairly certain that the plants in question have not been cross-pollinated by insects (which would happen if several varieties of the same type of plants grew in the same area) either. Such saved seeds might grow into something similar to the parents, or something tough and tasteless. Tomatoes are self-pollinating. So if you avoid hybrid varieties you'll be able to grow the same tomato from seed saved from each plant next year, even if different varieties were grown close together. Pepper and eggplant flowers can be cross-pollinated by insects, so different varieties of these would have had to be separated by 500 feet this season for seed purity.
Cucurbits - crops such as squash, cucumbers, gourds and melons - need even more personal space. All of these garden favorites must be pollinated by insects, so unless close relatives (of the same species) have been separated by a half-mile or more, you'll get some kind of squash surprise if you grow the seeds. For example, if you are growing a zucchini squash ( Cucurbita pepo) and a butternut squash (Cucurbita moschata) in the same garden, you can save the seeds from each and expect to have your plants come up true to type when you plant them next year. But a zucchini and an acorn squash (which is also Cucurbita pepo) in the same garden will cross, thanks to pollinating insects, and the seeds probably won't produce a replica of either parent plant. What you grow will look different and it could taste awful. (If saving seed from a variety of squash, melons and cucumbers is a major goal of your life, you can plant different kinds together and still save their seeds next year if you take over the insects' role and hand-pollinate the flowers.
Mother Earth News, Sep/Oct 1987
Bring new pleasures and superior plants to your garden
By Nancy Bubel
To prevent unwanted cross-pollination of those crops that do cross readily, you can either keep blossoming plants at recommended safe distances from each other (isolation in space) or plan your plantings so the different crops don't bloom concurrently (isolation in time). You could, for example, keep seed from both an early and a late corn, because the two plantings would tassel at different times. A third way to control pollination is to cage blossoming flowers in fine wire or netting to keep out unwanted influences. (You'd need to do the pollinating yourself or put a piece of maggot-infested meat inside to produce flies for pollinating.) An often easier alternative is to bag and hand-pollinate individual flowers.