Pollination in cherry trees can be confusing since two different species of trees are required for fruits. The sweet cherry (Prunus avium) is grown in U.S. Department of Agriculture plant hardiness zones 5 though 8. Sweet cherry trees are considered self-unfruitful and must be in close proximity to other trees for pollination and good fruit set. In contrast, sour or pie cherry (Prunus cerasus) trees are self-fruitful, meaning one lone tree can yield fruit. They also are more cold hardy, growing well in USDA zones 3 through 8.
Factors Affecting Pollination
Insects, especially bees, pollinate all cherry trees. Cherries bloom in early spring at a time when temperatures are cool and weather often brings precipitation. Wet, cold and windy weather at the time of flowering can limit the number of honeybees present for pollination. Cherry orchard owners often rent beehives to place within their tree fields to ensure better pollination among trees. If cherry blossoms are killed by an untimely spring frost, no pollination occurs, and the fruit crop fails to develop that year. Sweet cherries bloom earlier in spring than sour cherries.
Sour Cherry Cultivar Compatibility
Sour cherry trees may be planted singly or in groups. While fruiting occurs readily from pollen within their own flowering branches, an additional sour cherry tree nearby may boost fruit yield, but isn't necessary. Names of sour cherry cultivars include Montmorency, Hansen, Surefire, North Star, Balaton, Meteor, English Morello and Early Richmond. According to "Sunset Western Garden Book," any sour cherry tree can provide pollen for blooming sweet cherries, but it's not an ideal orchard management strategy.
Pollinating Sweet Cherries
Sweet cherry trees must grow near other sweet or sour cherry trees that bloom at the same time. Cultivars Index, Lapins, Skeena, Sweetheart, WhiteGold, Sonata, Stella, Symphony, Sunburst, and BlackGold are sweet cherry trees that can produce their own fruits from their blossoms and act as pollinators for other unfruitful sweet cherry varieties. Growing them in multiples also increases their own set of fruits. Sometimes these ten cultivars are called universal pollinators. Bing, Lambert, Royal Ann/Napoleon sweet cherry trees must grow near any universal pollinator tree for fruits to develop.