Jul/Aug 2008Hocus Pocus: pollination myths and misconceptionsby David L. Green

More complex and variable than most people realize, ideas about pollination suffer from many misconceptions and myths. Even among the experts, ideas about pollination may be vague or oversimplified.

Some gardeners hear the standard admonition "plant two varieties near each other for pollination," and never give a thought to bees. Pollen may be blown from one plant to another, as with pecans, for example, but it never will jump from one holly bush to another, because holly pollen, and that of many other cultivated plants, is heavy and sticky; it's adapted for insects to carry, not the wind.

A nursery catalog may say "plant A" is a good pollinator for "plant B." That really muddies the waters in people's minds. Plants cannot be pollinators; bees are pollinators - the agents that carry the pollen. One plant is the sire or pollenizer - the supplier of the genetic material for the next generation - for another plant.

One very common misconception is to envision pollination as a binary (off/on) act. This is why most gardeners don't identify pollination as the problem when their squashes "set" but fail to grow to maturity. The fruit has many seeds, each seed originating from an ovule in the ovary of the female blossom, which is fertilized by the male genetic material from pollen grains. When grains of pollen are deposited on the sticky end of the pistil, each grain grows a pollen tube down to an ovule to fertilize it.

If no pollen is deposited, of course no seeds will develop and the fruit will abort. But here's the kicker - some grains may be deposited, but not enough to fertilize all the ovules.

Suppose there are a possible 250 seeds and only 75 grains of pollen arrive at the flower. As the ovules become fertilized, they give off the chemical signals to develop the fruit. But if only a small percentage are fertilized, the chemical signal will only be strong enough to start the process, not to carry it to completion. The poorly pollinated part of a fruit is also more susceptible to fungi, which can disguise the real reason for the fruit abortion. Or the fruit may develop unevenly, the well-pollinated section developing fully, while the section with few fertilized seeds is small, deformed or shriveled. Such effects are easily seen in watermelons and cucumbers.

A general principle for multi-seeded fruits is that a high quality fruit usually does not result from a single visit by a pollinator such as a bee. Pollination is a progressive, not binary act, and pollinators must visit a plant multiple times to deliver the needed amounts of pollen.


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