Cross pollination is ordinarily not a problem for vegetables grown from their leaves, such as spinach, cabbage and greens of one sort or another. And there are few pollination problems with carrots, beets, radishes and vegetables grown from roots. However, some vegetables experience cross-pollination problems that can often be quite severe.
These vegetables produce flowers that are fertilized by their own pollen. Their flowers usually contain both the male and female parts. These plants don’t need insects or wind to be pollinated properly. Tomatoes, peas, lima beans, bush and pole beans, and lettuce are examples of self-pollinators.
Cross-pollinating vegetables need pollen from another plant in order to produce seed. The pollen is usually carried by the wind or by bees and other insects. The wind pollinates vegetables like chards, corn, spinach and beets. Bees, butterflies and other insects pollinate asparagus, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, celery, collards, cucumbers, eggplant, gourds, kale, kohlrabi, muskmelons, mustard, okra, onions, parsley, parsnip, hot pepper, pumpkin, radish, rutabaga, spinach, squash, turnips and watermelon.
This is when pollen from a different strain or variety of vegetable is introduced. This is how hybrid vegetables are developed. Seeds from hybrids usually revert back to the traits of their ancestors.
Varieties of winter squash, jumbo pumpkins and ornamental gourds that are closely related will cross-pollinate if you plant them close together. Don't worry about cucumbers or melons. Although tomatoes are self-pollinating, they can cross pollinate.
Preventing Cross Pollination
You do not have to worry about cross pollination if you buy seed for each growing season. Likewise, cross pollination between different kinds of vegetables is not a concern. If you save your seed, you have to worry about cross pollination between two varieties of the same vegetable. Self-pollinating vegetables are less susceptible to cross-pollination, but it can still happen. If your vegetables are pollinated by the wind, you can isolate them and hope that space will act as a barrier. Corn is an example of a vegetable that you can separate by space. Plants that are pollinated by bees, butterflies and other insects are more difficult to prevent from cross pollinating. Prevent insects from cross-pollinating your plants by putting them under a plastic tent and pollinating them by hand.
Remember that you need bees to pollinate many if not most of your vegetables. Do not use an insecticide to kill bees to prevent cross-pollination.