Fruit tree pollination
Pollination of fruit trees is required to produce seeds with surrounding fruit. It is the process of moving pollen from the anther to the stigma, either in the same flower or in another flower. Some tree species, including many fruit trees, do not produce fruit from self-pollination.
The pollination process requires a carrier for the pollen, which can be animal, wind, or human intervention (by hand-pollination or by using a pollen sprayer). Cross pollination produces seeds with a different genetic makeup from the parent plants; such seeds may be created deliberately as part of a selective breeding program for fruit trees with desired attributes.
Most apples are self-incompatible, that is, they do not produce fruit when pollinated from a 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 usually 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, and arrangements may be made with a commercial beekeeper who supplies hives for a fee.
|number of honey bee
hives per acre
for optimal pollination
|Apples (semi dwarf)||2|
Orchard mason bees are also used as supplemental pollinators in commercial orchards. According to Christopher O'Toole's The Red Mason Bee, Osmia rufa is a much more efficient pollinator of orchard crops than the honeybee. Home growers may find these more acceptable in suburban locations because they do not sting. Some wild bees such as carpenter bees and other solitary bees may help. Bumble bee queens are sometimes present in orchards, but not usually in enough quantity to be significant pollinators.
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. 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. Multiple bee visits are usually required to deliver sufficient grains of pollen to accomplish complete pollination.
The blossoms of all California Almond varieties are self-incompatible, requiring cross-pollination with other varieties to produce a crop. The single most important factor determining a good yield is pollination during the bloom period. More than a million colonies of honey bees are placed in California Almond orchards at the beginning of the bloom period to pollinate the crop. California beekeepers alone cannot supply this critical need, which is why honey bees travel across the country to the San Joaqin Valley each year. Although the recommended number of hives per acre is 2 to 3, due to the high demand in conjunction with the reduced availability of commercial beehives, many almond growers have to make do with a lower hive density during pollination. These growers started using semiochemical formulations, like SPLAT Bloom, to compensate for the low hive density. SPLAT Bloom manipulates the behavior of the bees, inciting them to spend more time foraging, and thus pollinating flowers in entire the almond orchard (increasing pollination and fruit set), not only close to the hive.
Pears are similar to apples, with the notable exception that pear blossoms are much less attractive to bees, due to lower sugar content than apple or contemporaneous wildflower nectar. Bees may abandon the pear blossoms to visit dandelions or a nearby apple orchard. There are three methods used that commercial growers use to compensate for this low attractiveness of pear flowers. One is saturation pollination, that is to stock so many bees that all area blossoms are worked regardless of the attractiveness to the bees. The second is to delay the movement of the beehives into the orchards until there is about 30 per cent 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 third method is to use semiochemical formulations, like SPLAT Bloom, to manipulate the behavior of the bees, inciting them to spend more time foraging in the pear orchard, thus increasing pollination and fruit set. The recommended number of hives per acre is 1.
Many citrus varieties are seedless and are produced parthenocarpically without pollination. Some varieties may be capable of producing fruit either way, having seeds in the segments, if pollinated, and no seeds if not.
Citrus that requires pollination may be self-compatible, thus pollen must be moved only a short distance from the anther to the stigma by a pollinator. Some citrus, such as Meyer Lemons, are popular container plants. When these bloom indoors, they often suffer from blossom drop because no pollinators have access. Hand pollinate by a human pollinator is a solution, though it is important to learn A few citrus varieties, including some tangelos and tangerines are self-incompatible, and require cross pollination. Pollinizers must be planned when groves are planted. This last group generally requires the addition of managed honeybee hives at bloom time for adequate pollination.
- Christopher O'Toole, The Red Mason Bee, ISBN 0-9539906-8-0, page 26
- "ISCA Technologies: A Leader of Innovative Pest Management Tools and Solutions". iscatech.com.