Cross-fertilization or cross-pollination occurs when pollen from a different but related plant fertilizes a seed. For some plants, such as many fruit trees, cross-pollination is essential for producing a good yield of fruit, according to University of Missouri Extension. In other instances, such as when collecting your own seed, cross-pollination is less desirable.
Fruits enclose a plant's seeds, and the development of seeds requires fertilization. When sexual reproduction doesn't occur, most plants won't produce fruits. Fertilization occurs when pollen containing sperm cells transfers to the female parts of a flower.
Cross-pollination and self-pollination refer to the respective fertilization by different or identical varieties of plants. Some plants are only fertilized via cross-pollination. As retired biology professor John W. Kimball points out, some flowers have genetic or biochemical barriers that prevent them from pollinating themselves, thereby increasing diversity through sexual reproduction. These plants--which include turnips, broccoli and roses--are physically unable to pollinate themselves and require cross-pollination.
From the plant's perspective, cross-pollination ensures genetic diversity, allowing possibly beneficial adaptations to spread. As a gardener, cross-pollination has advantages for you too. Even if you own a tree or plant that is able to self-pollinate, cross-pollination often produces a better yield, according to the University of Missouri Extension.
If you plan to save seeds from your vegetable garden this year, cross-pollination is less desirable. Related varieties of plants are often able to cross-pollinate, producing seeds that grow into offspring that bear little to no resemblance to the parent plants. The West Virginia University Extension Service recommends separating crops to ensure pure seed. However, some popular varieties such as squash and cucumber require a half-mile between crops to ensure cross-pollination won't occur, which is impossible for many home gardeners. "Rodale's Ultimate Encyclopedia of Organic Gardening" recommends covering blossoms with a paper bag before they open, then pollinating by hand.
Particularly when planting fruit trees, you should know whether a particular plant requires cross-pollination. Also consider which varieties will cross-pollinate with each other, including whether blossoms are open at the same time. Commercial growers often bring in bees or encourage pollination by keeping flowering weeds under control, encouraging insects to pollinate the trees instead.