Flowers but no fruit? Suspect pollination problems

When blooms appear on our fruit trees or vegetable gardens, we happily anticipate a bountiful harvest. If the bees help by doing their pollinating job, the fruits and vegetables should begin to develop. Pollinating, though, can get complicated. The pollinators need to be present and obliging, and the right flowers need to bloom at the right time.

Pollination occurs when pollen from a stamen, or male part, of a flower is transferred to the pistil, or female flower part. Germination begins when the pollen reaches the ovary of the flower and fruit development follows. Most flowers, however, need help getting the pollen onto the pistil and that’s where the pollinators come in. While we commonly think of bees and other insects as pollinators, pollen is also transferred by wind, water, birds, bats and small mammals. Pollination is the unintended consequence when a bee or butterfly moves from flower to flower to gather pollen or sip nectar and drops off extra grains of pollen in the process.

2 types of pollination
Although there are many types of pollinators, there are only two types of pollination — self-pollination and cross-pollination. A flowering plant is considered self-pollinating if the whole process can occur within one flower. Transfer of pollen from stamen to pistil may only require some air movement. Lettuce, peas, beans, tomatoes, eggplant and peppers are self-pollinating.

Cross-pollination requires the transfer of pollen from the stamen of one flower to the pistil in another flower. In some edibles, the flowers need to be on different plants. Spinach, beets, carrots, corn, onions, cabbage and squash are cross-pollinating. Most fruit trees are also cross-pollinating, and many require pollination between two or more trees, sometimes of a different variety. A lone apple tree, for example, may produce little or no fruit if there is no other apple variety close enough to provide bee or wind-carried pollen.

Cross-pollination becomes even more complicated in plants that produce separate male and female flowers. Plants in the Cucurbitaceae family, which includes cucumbers, squash and melons, are in this grouping. Typically, the male flowers appear first followed in a week or so by the female flowers. The female flowers can be recognized by the swelling at the base of the flower that looks like a miniature fruit. The female flowers are only open for one day so there need to be enough male flowers around for pollination to occur. Bees do the major work of transferring the pollen from the male to the female flower because the pollen tends to be too heavy or sticky to be moved by wind.

Pollination is not all or nothing. Unpollinated flowers will die and drop without forming fruit, however incomplete pollination can also occur resulting in fruit that is deformed or drops off early. This happens when insufficient pollen reaches the flower ovary. It takes multiple honeybee visits per flower to adequately pollinate many edible crops.

Pollinating by hand
If flowers are appearing and no fruit begins to develop, it could be time to try hand-pollinating. This can be done using a small, soft bristled paint brush to gently transfer pollen from the flower stamen to the pistil.

The primary causes of pollination problems are adverse weather conditions and lack of pollinators. Too much rain and cold, lack of sun or extreme heat can reduce the formation of healthy flowers and discourage pollinators. Bees, particularly honeybees, do not like cool, cloudy weather. Conversely, when temperatures are too high, pollen production decreases, flowers are damaged, fruit development slows and bee activity decreases.

Successfully growing edibles means also caring for the pollinators. Most pollinators have a longer lifespan than the flowers of a single type of plant, so a garden with a variety of plants that provide pollen and nectar throughout the growing season encourages pollinators to stick around. Because few pesticides are completely safe for pollinators, it’s best to avoid using them as much as possible.

Sponsored by UC Cooperative Extension, the University of California Marin Master Gardeners provide science- and research-based information for home gardeners. Email questions to helpdesk@marinmg.org. Attach photos for inquiries about plant pests or diseases. The office is closed for drop-in visits. Subscribe to the Leaflet, UC Marin Master Gardener’s free quarterly e-newsletter, at marinmg.ucanr.edu