Fruit tree pollination
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Apple pollination
Apples are self incompatible and must be cross pollinated. 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, 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. Orchard mason bees are also used as supplemental pollinators in commercial orchards. 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 mishapen apples, and slowness to ripen. The seeds can be counted to evaluate pollination. Well-pollinated apples are the 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. It generally requires multiple bee visits to deliver sufficient grains of pollen to accomplish complete pollination.
Citrus pollination
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 pollination by a human pollinator is a solution, though it is important to learn whether the variety is self fertile or self incompatible.
A few citrus varieties, including some tangelos and tangerines are self incompatible, and require cross pollination. Pollenizers must be planned when groves are planted. This last group generally requires the addition of managed honeybee hives at bloom time for adequate pollination.
See also [1] (http://edis.ifas.ufl.edu/BODY_AA092) for more extensive and specific information on citrus pollination.