Vegetables are pollinated in two basic ways: self pollination and cross pollination.
• Self pollinators are plants that produce flowers that are usually fertilized by their own pollen, commlonly when the male and female flower parts are contained within the same flower.
• Cross pollinators are plants with flowers that require pollen from another flower (a male flower on the same plant–thus a form of self-pollination–or from another plant) to produce a fertilized seed. Cross pollinators commonly require the help of insects or the wind to achieve pollination.
Self-pollinated vegetables include: bush and pole beans, lima beans, chicory, endive, lettuce, English and Southern peas, and tomatoes.
Wind-pollinated vegetables include: beets, chard, sweet corn, and spinach.
Insect-pollinated vegetables include: asparagus, broccoli, Brussels sprouts, cabbage, carrots, cauliflower, celeriac, celery, Chinese cabbage, collards, cucumbers, eggplant, gourds, kale, kohlrabi, muskmelons, mustard, okra, onions, parsley, parsnip, hot pepper, pumpkin, radish, rutabaga, spinach, squashes, turnips, and watermelon.
Vegetable Plant Pollination Basics
How a vegetable is pollinated is important if you want to grow plants that are true-to-type, meaning the same as the parent plant: for example, if you want the same fruit size, color, shape, and flavor, the same plant height or growing habit, the same days to maturity and harvest. If a vegetable is cross pollinated by a plant that is not the same strain or variety, it will not grow true-to-type.
Self-pollinated vegetables usually grow true. There is minimal risk of cross-pollination because the pollen from the male part of the flower (called the anther) usually falls directly on the female part of the flower (called the stigma) with little room for error or cross pollination.
Cross-pollinated vegetables often grow true-to-type but not always. Cross-pollinated crops may not grow true if the pollen from a different strain or varietyof the same crop or different crop in the same family is introduced into the female flower. When this happens the results can be unpredictable and the offspring will not grow true-to-type; the new seed and plant will not be like its parents. To avoid cross-pollination of unlike varieties or crops, flowering plants must be separated or isolated so that insect pollinators and the wind do not inadvertently cross pollinate the plants.
Often you will read or hear a discussion of open-pollinated and hybrid vegetables and other plants.
Open-pollinated plants. Plants that pollinate naturally–either by self pollination or cross pollination by wind or insects–are called open pollinated. (The opposite of open-pollination is controlled pollination–usually where the pollen comes from a different strain or variety or species by design and manipulation. Controlled pollination is plant breeding, commonly called hybrid pollination. Usually hybrid pollination comes after parent plants are selected for desired traits.)
Open-pollination usually, but not always, produces offspring similar to the parents and true-to-type–offspring that share the same traits of their parents. But because open-pollination is not closely controlled, open-pollinated plants may naturally pick up characteristics that differ from their parents. This is called biodiversity.
Self-pollenizing plants usually breed true even when open pollinated because the male and female parts are in the same flower or on the same plant. There is a decreased opportunity for pollen from unlike strains or varieties to reach the female flowers.
Cross-pollenizing plants will breed true when the pollen comes from the same plant or from the same strain or variety of plant. The challenge of growing cross-pollenizing plants by open pollination is preventing the pollen of unlike strains, varieties, or species from pollinating the female part of the flower.
One way to insure that cross-pollenizing plants do not breed untrue is to isolate cross-pollenizing plants at flowering time. Different strains, varieties, or species of cross-pollenizing plants can be isolated by distance (making it difficult for pollinating insects or the wind to transfer pollen) or enclosure–using insect proof cages to prevent cross-pollination. Separation by distance in the home garden is difficult if not impossible; most cross-pollenizing plants need to be separated by ¼ of a mile or more to avoid cross-pollination by wind or insects.
A useful home garden strategy for avoiding cross-pollination of open-pollinated plants of differing strains, varieties, or species is to sow seed or set out transplants of differing plant strains or varieties in the same family (two different varieties of cabbage family plants, for example) at different times–usually two weeks or more apart–so that the plants flower at different times. That way insects or the wind can not achieve cross pollination. Self-pollinated crops of different strains or varieties can be separated by a both time and a few rows.
Biennials. Not all vegetables are pollinated in the same growing season they are planted. Biennials vegetables flower in their second year of life (providing they survive the winter in between). That means pollination of biennials occurs in the plants second year of life. Biennial vegetables include: beets, Brussels sprouts, cabbage, carrots, celeriac, celery, chard, collards, Florence fennel, kale, kohlrabi, leeks, onions, parsley, parsnips, winter radishes, rutabaga, salsify, and turnips. Perennial vegetables also do not necessarily flower the same year that they begin growing.
Hybrids. Hybrid plants are the result of the deliberate cross-pollination of two selected lines of plants. Hybrids are commonly produced to make seed that combines the best characteristics of the parents–productivity, uniformity, vigor, pest resistance. Seeds collected from hybrids usually do not grow true to their hybrid parents; rather they often revert and display characteristics from more distant progenitors.
F1 hybrid seed. Many selectively bred vegetable varieties are F1 hybrids (Filial 1, the frist filial generation). F1 hybrids are made by crossing two distinctively different parent lines or parent cultivars to produce an offspring that is different from the parental types but with specific desirable characteristics from either or both parents. F1 hybrids are created for exceptional vigor and improved growth, quality, uniformity, and yield. Seed from F1 hybrids will not breed true-to-type. F1 hybrids must be remade by controlled pollination every time seed is needed. (That is why F1 hybrids can be expensive.) This is a time consuming and exacting breeding process not usually undertaken by home vegetable gardeners.