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Vegetables: Pollination

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Pollination, the transfer of pollen within a flower or between flowers, is needed for many vegetables to produce. With vegetables we grow for their leaves (greens, spinach, cabbage) and roots (beets, carrots, radishes), pollination is not important. But with vegetables we grow for their developing fruit, ripened fruit, or seeds (melons, corn), pollination is almost always needed.

Pollen is produced in the anthers (male parts) and must be moved to the pistil (female part). One part of the pistil, the ovary, develops into the seed or fruit that is eaten—squash, tomatoes, cucumbers, corn kernels. Pollen is moved from the anthers to the pistil in one of three ways.

Corn pollen is carried by the wind as it falls from the tassel to the silks of the ears. If anything prevents this wind transfer of pollen, the result is ears with empty rows and missing kernels. Corn planted in a single row loses most of its pollen. This is why corn should be planted in a block of adjacent rows rather than one or two very long rows. High temperatures and drought do not interfere with the transfer of corn pollen but can prevent proper pollination and fertilization, resulting in poorly developed ears.

The pollination process in all beans, peas, and tomatoes is called self-pollination because the transfer of pollen takes place within the individual flowers without the aid of insects or wind.

Squash, pumpkins, melons, and most cucumbers are insect-pollinated. In these vegetables, which have the male and female flower parts in separate flowers (yet still on the same plant), insects transfer pollen from male flowers to female flowers while going from flower to flower, collecting nectar and pollen. The most common pollinating insects are honeybees and bumblebees.

Corn pollen is carried by the wind as it falls from the tassel to the silks of the ears.

Bees often are seen on vegetables that are wind- and self-pollinated, where they are collecting pollen and nectar. Since pollinating insects are so important in the garden, it is important to consider them when choosing and applying insecticides. Choose insecticides that are least toxic to bees, and apply them late in the day when bees are not actively working in the garden.

Vegetables that are self- and insect-pollinated often suffer from lack of pollination and fertilization, just as wind-pollinated corn does. High temperatures, shade, and insufficient moisture often result in pollen that does not behave normally and causes a lack of fruit development. Poorly shaped fruit (cucumbers, watermelons, tomatoes) result from incomplete pollination.

Crosspollination between different vegetables is an unnecessary worry of many gardeners. Different varieties of the same wind- and insect-pollinated vegetables may cross, but there is no crossing between the different vegetables: cucumbers, melons, and squash. All summer squash, Halloween pumpkins, vegetable spaghetti, acorn squash, and small ornamental gourds are closely related and do cross if planted close to one another. This is of no concern to gardeners who do not save their own seed. Jumbo pumpkins and most winter squash can cross. If you grow several varieties of summer and winter squash and pumpkins in the same garden, purchase fresh seed each year.

The different corn colors (yellow and white) and types (normal, sugary enhanced, supersweet, field, and pop) crosspollinate if planted close to one another, and if they silk and tassel at the same time. Results can vary from a few yellow kernels on normally all-white ears to a situation where the corn is not fit to eat. All sweet corn must be isolated from field and popcorn, and all supersweet corn must be isolated from all other corn.