Experts tackle readers’ garden questions.
Q: We live in Sacramento and have two cherry trees that did not produce one cherry this year. They have in the past. We wonder why. We also wonder why our fig always produces lots of sour figs. We keep saying we will give it another year. Maybe this year should be its last unless you have some good ideas.
David and Arlene Grimm, Sacramento
Master gardener Rachel Tooker: There are a number of reasons why fruit trees fail to set fruit. Reasons include lack of pollination, frost damage, alternate bearing, lack of sunlight, improper pruning, nutrient deficiencies, improper irrigation and disease or pests.
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Cherry trees require cross-pollination from a different cultivar (variety) within the same species, so having at least two trees each of a different variety or grafting a different cultivar into the same tree is required for good pollination. In addition, bad weather conditions during pollination can cause poor fruit set. Rain during bloom, strong winds and cold temperatures can greatly reduce or prevent bee activity and wind pollination.
Beyond its impact on pollination, weather can cause other difficulties. When exposed to freezing temperatures, flower buds can fail to develop and may drop off. Open blossoms can die if the temperature falls below 27 degrees. In addition, a warm winter can cause insufficient hours of temperatures below 45 degrees (winter chilling hours), which can lead to poor flower development, decreased fruit set due to a weak, prolonged bloom and, in some cases, flower bud drop. Cold temperatures above freezing (33 to 45 degrees) also may reduce fruit set. Temperatures above 104 degrees for several days also may cause flower and leaf buds to drop.
Another reason that trees fail to set fruit is called alternate bearing, which is when fruit trees produce a heavy crop one year and a light crop the next. Between the full bloom stage and the small fruit stage, most trees produce flower buds for the next year. A heavy crop in the year before may cause less vigor for the tree in the next year. Since cherry flower buds form the previous summer, an especially heavy crop during bud formation may prevent adequate flower buds from forming or may cause them to drop. Consequently, next year’s crop will be light. The most effective way to reduce alternate bearing and increase final fruit size is to hand-thin the fruit within 30 to 45 days after full bloom, when the fruit reach 3/8-inch to 1 inch in diameter.
Too much shade can be another culprit. As other plants become more mature, they may be causing more shade to fall on the cherry trees. Fruit trees do best in full sun (at least six hours per day). Too much shade limits light exposure and fruit set during the growing season.
Too much or too little pruning is another consideration. Because the buds of cherry trees form on 2-year-old spurs in the previous summer, severe dormant pruning in the winter can reduce most flower buds or lead to heavy leaf growth and poor fruit set. Conversely, lack of pruning may lead to less light penetration into the tree, causing decreased fruit set.
Lack of nitrogen also can lead to poor fruit set. If nitrogen deficiency is suspected, fertilizer should be applied in the summer to increase the nitrogen storage in the tree over the winter. The stored nitrogen is used for flowering and fruit set during the following spring.
Excessive or insufficient irrigation also can reduce flowering and fruit set, as well as quality of fruit. Fruit and nut trees require consistent and uniform soil moisture. Water stress during fruit production can reduce yields and make trees more susceptible to boring insects or diseases, which also can weaken trees and result in poor fruit set. Moisture should be replaced well before the trees begin to show stress. Avoid frequent light watering with sprinklers as this creates a shallow root system. Use a garden hose, soaker hose or drip system for a good deep watering every two to three weeks, irrigating for 12 to 24 hours to achieve a 3- to 6-foot depth. Do not water established trees near the trunk and lower branches, as this promotes root and crown disease.
Regarding sour figs, it would be helpful to know the variety of the tree, which would determine the type of fig and its flavor. However, regardless of the variety, ripening figs can turn sour from yeasts brought into the fruit by insects, such as fruit flies and dried fruit beetles, that enter through the tiny hole at the bottom of the fig, referred to as the eye. Ripe fruit that is sour and fermented, and has fluid dripping from the eye, may be infested.
To manage the problem, remove and destroy any infected fruit, both attached to and fallen from the tree. Early harvest and orchard sanitation can help reduce the damage caused by pests, as well as planting less susceptible varieties, such as Mission, which have small eyes that are harder for insects to penetrate.
In addition, trapping the dried fruit beetles can be effective in reducing the population if done before the fruit ripens and becomes attractive to them. Once the fruit begins to ripen, insecticides may be necessary, but it is critical to follow label directions.
For more information on fruit-setting, check out the University of California Cooperative Extension publication “Why Fruit Trees Fail to Bear,” available free online. For more information on watering fruit trees or dried fruit beetles in figs, go to the UC Integrated Pest Management (IPM) website.
Rachel Tooker is a UC Cooperative Extension master gardener for Sacramento County.
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