Gardeners enjoy the beauty and fragrance of flowers, but from a plant’s standpoint flowers have one very basic and essential function – to ensure the production of seeds and thus the next generation. For a plant to produce seeds its flowers must be pollinated, either through self-pollination or cross-pollination, and most plants have evolved to primarily use one strategy.
What is pollination?
Pollination occurs when a grain of pollen, containing one-half the genetic material needed to create a new seed, is transferred to a flower’s ovule, the egg-containing part of a flower, which provides the second half of DNA required for a viable seed to form. Pollen is the contribution of the male parent and the female parent provides the ovule.
Self-pollinated crops, such as beans and peas, have evolved strategies to ensure self-fertilization. Often their flower structure prevents the easy movement of pollen outside the flower or flowers may be pollinated before they even open!
Self-pollination often results in homozygous plant lines, or plants with identical gene pairs for many traits, which ensures new plants grown from saving seed will be almost identical to the parent plants. Beans and peas are two examples of self-pollinated plants in the home vegetable garden.
Cross-pollination occurs when pollen from one plant fertilizes the ovules of another plant. Some plants cannot set viable seed without cross pollination, as is the case with broccoli and many apple cultivars. Cross-pollination happens through the actions of pollinators, such as insects and other animals, or by wind blowing pollen from plant to plant. In the home vegetable garden, tomato, watermelon and cucumbers are cross-pollinated by insects and sweet corn is wind pollinated.
How pollen affects flowers, fruit
The pollen source, whether self- or cross-pollinated, does not affect the look or flavor of a new fruit, such as an apple, tomato or watermelon. The part of the fruit we eat originates from the female plant’s ovule and has the female’s genetic traits no matter what the male pollen source. However, seed saved from cross-pollinated plants results in a varied array of fruit characteristics in the next generation due to the mixing of female and male genetic traits.
If the plant part we eat is a vegetative structure - like broccoli, lettuce, beets or potatoes – the plant’s characteristics are dictated by the genetics of both the male and female parents. Saving seed from year to year will not reliably provide new plants with the same characteristics as the original parent.
But in some cases, the part of the fruit we eat is actually the seed, such as beans, peas or kernels of sweet corn. Seed characteristics – size, color, texture, flavor – are affected by the pollen source. Beans and peas are self-pollinated, so introduction of a new male pollen source is uncommon.
However, sweet corn, cross-pollinated by wind, is very susceptible to changes caused by pollen source. All types of corn readily cross-pollinate. Sweet corn must be isolated from field corn, popcorn and ornamental Indian corn either by location or flowering dates, or the ears harvested will have kernels of different types. A distance of 400 yards or planting so that maturity dates are one month apart, is necessary to create isolation and prevent cross-pollination between cultivars.
White and yellow sweet corn types also cross-pollinate, but the effects are minimal if they all have the same sweetness gene (sh2 – supersweet or shrunken, SE - sugary enhanced or su - sugary). Corn cultivars with the SE gene may benefit from isolation from other corn sweetness types, but it is not absolutely necessary.
Each summer, Extension offices receive calls from clientele wondering if their muskmelons have crossed with cucumbers because the muskmelon flavor is poor. This is impossible because cucumbers, Cucumis sativus, and muskmelons, Cucumis melo, cannot cross-pollinate each other. The Cucurbit family is unique in that plants in different species cannot cross-pollinate. Muskmelons develop the best flavor and aroma when they ripen under warm, dry weather. Unripe melons, or those grown under cold, wet conditions may have poor flavor and sometimes taste like cucumbers but are not due to cross-pollination.
Neither are the weird-shaped pumpkins, squash or gourds appearing in many gardens each year. These fruits are usually the result of plants grown from seeds saved from last year’s fruits, volunteer plants from fruit (possibly diseases or broken ones) left in the garden the previous year, viruses or rarely an off-type seed mixed into a seed packet. The quality and appearance of pumpkin fruits and other squash growing in the garden are not affected by pollen source.