As humans, it’s hard to imagine sex without the ability to move, which is the fundamental challenge of plant pollination. To make seeds, reproductive cells (called “gametes”) from separate male and female cells must join, often from separate flowers and sometimes from separate plants — but the plants can’t move to help make the miracle happen. Instead, they rely on the resources around them: wind, insects, birds and, sometimes, people.
Understanding plant pollination is more crucial than ever, as we are in the midst of a severe decline of native pollinators because of ubiquitous pesticide use in monocrop agriculture. While we may not be able to change this unsustainable practice overnight, we can
Like most plants, food crops have sophisticated reproductive systems in place, ready to take on the pollination gamble. Delicate female parts are hidden inside the flower, safe from the elements. Plant pollen faces an out-in-the-open, hostile journey — even with a waxy coating and a stash of carbs for energy, male pollen grains must quickly find their way to a receptive female organ. There, they grow a tube that unites them with the ovary, thus forming a fertilized seed.
Beans, peas and tomatoes are examples of species that are “self-fertile,” which means that all the necessities for successful pollination reside within each flower. A bit of well-timed shaking is all that’s needed to sprinkle pollen grains where they need to go, which in nature is done by wind and visits by buzzing insects. When beans, peas and tomatoes bloom but set no fruit, the weather — not a lack of pollinating insects — is usually to blame. Cold or hot weather often causes flower abnormalities that, in turn, cause fertilization to fail. To overcome weather-related crop failures, plant a few varieties known for their cold or heat tolerance — for example, ‘Glacier’ and ‘Tropic’ tomatoes. These are more likely to set fruit successfully under conditions that would cause a sensitive variety, such as ‘Mortgage Lifter,’ to shed most of its blossoms.
To produce fruit, about 35 percent of food crops require repeat visits by insect pollinators, such as bees, wasps, butterflies, flies and beetles — any creature that will crawl into the blossom, transfer pollen from the male anthers to the female stigma, and then move on to do the same to another flower. The pollen grains of insect-pollinated crops are sticky for easy transport, and many plants advertise for pollinator services with colorful flowers — visual billboards promising a sweet drink of nectar, some nutritious pollen and occasionally a safe place to sleep. Case in point: Male squash bees hide away inside bright orange squash flowers late in the day, and then the blossom closes up around the sleeping bees as night falls, protecting them from predators. Inside, the bees’ movements ensure that pollen spreads throughout the flower and all over the bees. In the morning, the guys are rested and the squash flowers are nicely pollinated.
Most flowers have not one but dozens — or even hundreds — of ovaries that need to be fertilized, so pollination usually takes place over two to three days. In any seed-bearing fruit or vegetable, each seed represents a fertilized ovary. All the little seeds on strawberries result from 16 to 25 visits by pollinating insects. In comparison, because of differences in flower design, only half as much pollinator activity is needed to produce a watermelon containing hundreds of ripening seeds.
We recently asked the MOTHER EARTH NEWS Facebook community to dish on pollinator activity in their gardens. People with beehives (or, in the northernmost United States, more cold-hardy mason bee boxes) said pollination couldn’t be better, and gardeners who went out of their way to grow borage, catnip and other bee-friendly flowers also reported ample buzzing in their gardens. Alarmingly, however, many long-term growers from about a dozen different states reported seeing fewer and fewer bees and other pollinators year after year. Some said they have been forced to hand-pollinate squash in recent years in order to get a harvest. Find detailed techniques for encouraging native pollinators in How to Attract Native Bees to Your Organic Garden. The most effective strategy is simple: Grow as many nectar-producing flowers as you can fit into your garden, because pollinators can’t resist the sweet, nutritious treat of nectar.
Wind-pollinated plants don’t need insects to move pollen to where it needs to go. Instead, they produce large amounts of smooth, dry pollen grains befitting of air travel. Corn, wheat, and other grains and grasses are primarily wind-pollinated, as are crops in the spinach family. Pollination won’t affect your spinach harvest because you’ll eat only the leaves — but it would affect any seed-saving efforts.
For wind pollination to work well, several plants must grow close enough together that pollen gets released in drifting clouds. That’s why corn should always be planted in blocks rather than in long, single rows. With corn, every silk that emerges from the tip of the ear leads to an ovary, or future kernel. To form a well-filled ear, fresh pollen grains must land on hundreds of silks and then successfully germinate and grow long tubes to hundreds of receptive ovaries. Rather than risk a poor harvest, many gardeners with small corn plantings hand-pollinate to make sure they get completely developed ears.
Wind-pollinated crops are susceptible to pollen pollution — for example, when pollen from field corn blows into plantings of sweet corn. Organic growers in commercial farming areas face the inflow of pollen from genetically modified corn. Windbreaks composed of trees or hedges of tall sunflowers or sorghum reduce incoming alien pollen, as does a planting distance of several hundred feet. Also try planting times that are at least two weeks earlier or later than those of nearby commercial farmers.
In small gardens, pollination of many crops may fall short because of too few insect-attracting flowers. If, for example, you have fewer than four plants of a cucurbit cousin — such as cucumbers, squash and pumpkins — or only a small plot of corn, you can greatly improve fruit set by performing simple hand-pollination techniques. Hand pollination is also helpful in areas where drought or pesticides have led to a shortage of insect pollinators.
A dry paintbrush or a feather is the only tool you’ll need to hand-pollinate strawberries and other plants that insects normally pollinate. For such crops, a few dabs with either implement will transfer pollen from the male to the female parts of each flower. For cukes and squash, you’ll need to gather a few newly opened male blossoms (which are borne on bare stems) and touch the pollen to the female flowers (which have a tiny round fruit at their base). To mimic bumblebees’ buzz-pollination of eggplants, peppers or tomatoes, hold the blossom or blossom cluster and then touch the back of a vibrating toothbrush or a bladeless electric razor to the stem for a few seconds.
Small plots of sweet corn will produce excellent ears if they’re hand-pollinated in the mornings, when pollen is fresh and silks are stickiest. Doing so requires no special equipment. Every day for a week, just pinch off pieces of tassel from the top of the plant and tap them over the moist, new silks of emerging ears. Do this when the tassels are fully open and beginning to shed yellow pollen. Or, on still mornings, give the stalks a sharp thump with your hand to release pollen.
For tree fruits, use a tuft of feathers attached to the end of a long stick to tickle each blossom cluster, branch by branch. Repeat daily for a week. For species that are not self-fertile, such as most apple trees, you’ll need to move pollen from one tree to another. Gather excess secondary blossoms from a pollinizer variety (a variety that produces sufficient pollen) in a paper bag, and dip the feathers in the bag of blossoms to then apply to the blossoms of the tree you’re pollinating. Or, try this: Crab apples are good pollinizers and can pollinate other apple trees. Cut a bunch of blossom-laden crab apple branches, put them in a 5-gallon bucket of water, and then place the bucket inside the canopy of another apple tree. Bees will visit the crab apple blossoms and transfer the pollen to your apple blossoms.
Discover how to make your garden a haven for bees and other pollinators in Plant a Pollinator-Friendly Garden.
Contributing editor Barbara Pleasant gardens in southwest Virginia, where she grows vegetables, herbs, fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website or finding her on Google+.