Fruit tree pollination

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Most apple plants/trees are self-incompatible, that is, they do not produce fruit when pollinated from a flower of the same tree or from another tree of the same cultivar, and must be cross pollinated. A few are described as "self-fertile" and are capable of self-pollination, although even those tend to carry larger crops when cross pollinated from a suitable pollenizer. A relatively small number of cultivars are "triploid", meaning that they provide almost no viable pollen for themselves or other apple trees. Apples that can pollinate one another are grouped by the time they flower so cross-pollinators are in bloom at the same time. Pollination management is an important component of apple culture. Before planting, it is important to arrange for pollenizers – varieties of apple or crabapple that provide plentiful, viable and compatible pollen. Orchard blocks may alternate rows of compatible varieties, or may plant crabapple trees, or graft on limbs of crabapple. Some varieties produce very little pollen, or the pollen is sterile, so these are not good pollenizers. Good-quality nurseries have pollenizer compatibility lists. Growers with old orchard blocks of single varieties sometimes provide bouquets of crabapple blossoms in drums or pails in the orchard for pollenizers. Home growers with a single tree and no other variety in the neighborhood can do the same on a smaller scale.

During the bloom each season, commercial apple growers usually provide pollinators to carry the pollen. Honeybee hives are most commonly used in the United States, and arrangements may be made with a commercial beekeeper who supply hives for a fee. Honeybees of the genus Apis are the most common pollinator for apple trees, although members of the genera Andrena, Bombus, Halictus, and Osmia pollinate apple trees in the wild.^[2] Solitary bees or wild bees such as ground-nesting mining bees (Andrena) may play a far bigger role in pollination than at one time suspected,^[3] and are alternative pollinators in orchards.^[4] Bumble bees are sometimes present in orchards, but not usually in enough quantity to be significant pollinators; in the home garden with only a few trees, their role may be much greater. Apple growers also rely on several species of wild bees for pollination in their orchard.^[5][6]

Increasingly Orchard bees (spring mason bees, genus Osmia) are being used in fruit tree pollination.^[7] According to British writer Christopher O'Toole in his book The Red Mason Bee, Osmia rufa is a much more efficient pollinator of orchard crops (in Europe) than honey bees.^[8] Both O. rufa and O. cornuta are used in Europe, while in western North America, the "Blue Orchard Bee" (Osmia lignaria, more black than blue in color) is a proven orchard pollinator.^[9] In Japan, the Japanese Orchard Bee—the hornfaced bee, Osmia cornifrons—provides up to 80% of the apple pollination.^[10] Beyond Japan, Osmia cornifrons is also used increasingly in the eastern US, because like other mason bees it is up to 100 times more efficient than the honeybee—a mere 600 hornfaced bees being required per hectare, as opposed to tens of thousands of honeybees.^[11] Home growers may find these more acceptable in suburban locations because they rarely sting.

Symptoms of inadequate pollination are small and misshapen apples, and slowness to ripen. The seeds can be counted to evaluate pollination. Well-pollinated apples have best quality, and will have seven to ten seeds.^[12] Apples with fewer than three seeds will usually not mature and will drop from the trees in the early summer. Inadequate pollination can result from either a lack of pollinators or pollenizers, or from poor pollinating weather at bloom time.^[13] Multiple bee visits are usually required to deliver sufficient grains of pollen to accomplish complete pollination.

Like apples, pears are self-incompatible and need to attract insects in order to be pollinated and produce fruit.^[1] One notable difference from apples is that pear blossoms are much less attractive to honeybees due to their pale coloring and light odor. Bees may abandon the pear blossoms to visit dandelions or a nearby apple orchard. The majority of pollinators of pear trees are honey bees, although pears are also visited by blow flies and hoverflies.^[1] In some areas, such as the Emilia-Romagna area of Italy, honeybees could not be used to pollinate pears due to the spread of fireblight in the 1990s, and Osmia cornuta were used as pollinators instead.^[14] A way to combat the low attraction of honey bees to pear blossoms is to use bee attractants to entice the bees to pollinate the flowers. Bee attractants may include pheromones that mimic the brood pheromone or the juvenile pheromone, or other attractants.^{[citation needed]} There are also other methods for attracting honey bees to pear blossoms. One is saturation pollination, that is to stock so many bees that all area blossoms are worked regardless of the attractiveness to the bees. Another method is to delay the movement of the beehives into the orchards until there is about 30 percent bloom. The bees are moved into the orchard during the night and will usually visit the pear blossoms for a few hours until they discover the richer nectar sources. The recommended number of hives per acre is one.

Most Asian plums require a second variety for cross-pollination, whereas most European plums are self-fertile. European plums that dry well are also called prunes, the common name for the dry fruit. Several of the Asian plums bloom early and pollinator management is similar to apricots. However the fruiting wood especially of Asian plums can be brittle and over-pollination requires aggressive thinning of fruits.

Apricots are botanically similar to almonds and like almonds they can bloom early, making pollinator management similar except that a tree can overcrop if the fruit is not thinned.

Most peaches and nectarines are self fertile and thus require fewer pollinators than apples and pears.

Pie cherries, also called sour cherries, are self-fertile. In some varieties, fruit yield could increased by utilizing pollinators.^[15] Most sweet cherries need a second variety for pollination, and even self-fertile varies have improved fruit-set with a second variety. Pie cherries can pollinate sweet cherries. Cherries do not need thinning.

Some figs produce fruit without pollination. Figs that are pollinated rely on fig wasps.