To enjoy fresh fruits and vegetables straight from the garden, we cannot ignore the importance of effective pollination.
For the many garden plants that produce edible seeds and fruits, pollination is essential. Plants like cantaloupe, tomato, and apricots will often abort fruit that has not been properly pollinated and seed producing plants like beans, almonds and pecans often end up with empty pods or shells when flowers do not receive their share of pollen. If plenty of fresh fruit and vegetables are important to us, it is critical that we know about, and correctly manage, the process of pollination.
Pollen is the male particle that carries genetic information used in the production of new plants. Pollen is produced in the anther or male reproductive structure of flowering plants, and is transferred, either by the wind, gravity, insects or other agents to the stigma, or female part of the flower. When the male information is combined with the information provided by the female part of the flower, the process of pollination is complete. Under normal conditions in native environments, pollination usually is a normal function of nature, and plants are able to progress flawlessly from generation to generation practicing the law of survival of the fittest.
Occasionally, however, problems can arise. In plants where the pollen from their own male structures fertilize the female structures in the same flower, the process is called self-pollination. Garden peas, wheat, oats and cotton are examples of common plants that self-fertilize and in most cases, good pollination usually occurs. Most pollen-related problems in self-pollinating plants are usually related to heat stress where high temperatures sometimes kill the pollen before a transfer of genes can occur. In home gardens, proper adherence to planting date recommendations will usually prevent pollination problems in self-pollinated garden plants.
Plants that require the transfer of pollen from one flower to another are dependent upon cross-pollination. This extra step can mean trouble. If the transfer is interrupted or prevented in any way, the development of fruit or seeds will simply not occur.
The exchange of pollen during the cross-pollination process is often facilitated by outside agents called pollinators. When we think of pollinators, most people think immediately of honey bees, and you would be absolutely correct. The honey bee is a most effective pollinator and most of the foods that we enjoy occur because of their hard work. Citrus, watermelons, apples and many more are all dependent in some way or another on honey bees.
However, these effective and hard working animals do not have the field, forest or garden to themselves. There are many other species of bees as well as other insects and animals that help spread pollen from plant to plant. Some of the more common pollinating agents besides the honeybee include bats on saguaro cacti; birds, especially hummingbirds on many long, tubular-shaped flower plants; and some wasps, flies, moths and butterflies. In addition to animal pollinators, we cannot forget that the wind also plays a major role in the cross-pollination of many plants.
Generally, plants with inconspicuous flowers which lack petals, odor, and nectar are pollinated by the wind. Grasses, cottonwoods, elms, walnuts, oaks, pecans and, yes, let’s not forget ragweeds, are all wind pollinated. Grouping desirable wind-pollinated plants together in blocks will help make sure that proper pollination occurs. Commercial pecan and pistachio orchards are planted with a row of pollinator trees every so often in the block to ensure good pollination.
A good garden example is sweet corn. Because it is wind pollinated, sweet corn must be planted in blocks and not long narrow rows in the garden to ensure the best coverage of pollen onto the developing ears. Poor pollination in corn will be seen as skips between the kernels at maturity. Also, similar symptoms can also be seen when temperatures above 100 degrees F kill pollen before pollination occurs.
Although wind-pollinated plants need not concern themselves with the business of attracting pollinators, it is a different matter for plants that need more than a breezy day to assure that there is a next generation.
Members of the Cucurbitaceae family, like squash, cantaloupe, and watermelon, require the actual movement of pollen from the showy male flowers to the inconspicuous female flowers where the fruit develops. These flowers are in completely different locations on the plant and reproductive success is entirely dependent upon insect pollinators like honeybees to make the transfer of pollen.
A pollination problem often shows up among cucurbits in the early spring. In the desert, we generally plant them early to obtain a yield before the hot weather shows up. Since bees are sluggish in cold weather, late winter storms arriving during the pollination period can slow down the movement of these pollinators and result in some flowers not receiving pollen. Squash and melons that turn brown at the tips and begin to shrivel up soon after fruit set are a direct result of poor pollination. There is nothing that can be done for these aborted fruit but not to worry. Just wait a little bit. As the weather warms, pollination gets back on track and plenty of fruit will set later.
Poor pollination can also cause problems in tomatoes. If the flowers do not properly get pollinated, the plant terminates the flower. During that process, the aborted flower develops a layer of thin, weak cells across the flower petiole. Eventually the weight of the flower causes the discarded flower to break off and fall from the plant. Since tomatoes are wind pollinated, sometimes a good shaking of the vine and the flowers will cause the pollen to shift inside the flower and help pollination.
Another important pollination concept is self-incompatibility. In all sweet cherries and in many varieties of apples, plums and pears, the pollen produced in anthers of one horticultural variety does not function on stigmas of the same variety, even though the stigmas are in flowers of a different tree. This self-incompatibility can significantly reduce or prevent the production of fruit during the growing season. It is for this reason, that pollinator varieties are often recommended for certain plants. For example, the ‘Anna’ apple will produce some fruit without a pollinator, but does much better when it is sharing pollen with another variety like ‘Golden Dorsett.’
Once we understand the importance of pollination, we understand the importance of the pollinator. I have to tell you that we definitely are worried about the future of the honey bee. If these friends are lost for any reason, we could be in serious hot water when it comes to food on the table. This is one of the reasons that we are super careful with insecticide applications. It is important to follow the label directions carefully and avoid applications when the bees are active.
By studying the biology of our beneficial plants, potential problems can either be headed off or at least understood when they occur.
If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande. The telephone is (520) 836-5221, 204.
The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.
Richard Gibson
Extension Agent, Agriculture
University of Arizona
Pinal County Cooperative Extension Office
College of Agriculture and Life Sciences
820 E Cottonwood Ln, #C
Casa Grande, AZ 85122
Tel: (520) 836-5221 ext. 227
Fax: (520) 836-1750
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