Apple pollination

Michele R. Warmund
Department of Horticulture
University of Missouri

All varieties of apple trees should be cross-pollinated with another apple or crabapple variety. To attain the best fruit set on apple trees, the king blossom (the largest and first one to open) in the flower cluster must be pollinated. Thus, the bloom periods of the pollinizer and the king blossom of the apple tree must overlap.

In backyard plantings, two semidwarf apple varieties that bloom at the same time should be planted within 50 feet of each other. Two dwarf apple varieties with similar bloom periods should be spaced less than 20 feet apart to ensure the transfer of pollen between trees (Figure 2).

Bloom period
 

Figure 2
Apple and crabapple bloom periods. The shaded area represents the time of bloom.

Although some apple varieties, such as Lodi, Liberty, Empire, Winesap, Jonathan, Jonagold, Gala, Golden Delicious, Rome and Granny Smith may be listed as self-fruitful, they will set more fruit on an annual basis if they are cross-pollinated. Additionally, some apple varieties, such as Winesap, Stayman, Mutsu and Jonagold, produce sterile pollen and therefore cannot be used to pollinate other apple varieties. Many nursery catalogues include pollinization compatibility charts (Figure 3) or recommend good apple varieties to use as pollinizers.

Apple pollination
 

Figure 3
Apple pollination. Except where indicated, varieties listed on the left can be used as reliable pollinizers for cross-pollination.

Manchurian crabapple, with profuse white flowers, is commonly used to pollinate early- to mid-blooming apple varieties, while Snowdrift crabapple is used for mid- to late-blooming apple varieties (Figure 2). When using a crabapple tree as a pollinizer, it should be planted within a similar distance to an apple tree as listed above.

In situations where a solitary apple tree is planted, branches of open fresh blossoms of another apple or crabapple pollinating variety can be placed in buckets of water and hung in the tree. Another way to ensure pollination where a single tree is planted, is to top-work or graft another apple variety onto the existing tree. To top-work an apple tree, 6- to 8-inch sections of branches of one apple variety are cleft-grafted onto terminal branches of another variety.

In commercial apple plantings, a row of pollinizer trees is often planted between every four rows of the main variety of trees (Figure 4). If pollinizers are placed within the row, every fifth semidwarf tree is a pollinizer and each pollinizer is offset in adjacent rows to stagger them throughout the orchard block. In high-density plantings of dwarf trees (5 to 6 feet between trees within the row), apple or crabapple pollinizers may be planted between eight to ten trees of another variety in the row.

Alternative planting
 

Figure 4
Alternative planting plans to ensure cross-pollination of apple trees.

Beehives are generally placed in commercial apple orchards as the king flowers open. If hives are brought in before this time, bees may forage flowers of other broad-leaved plants instead of the apple blossoms. For this reason, dandelion flowers should be removed by mowing or by herbicide treatment before hives are placed in the orchard. In orchards where semidwarf trees are planted, one hive of a medium-strength colony (15,000 to 20,000 bees) is generally sufficient per acre. Two hives per acre are used in high- density orchards where dwarf apple trees are planted. Extra strong colonies of as many as 50,000 bees have been effective in pollinating four acres of semidwarf trees under ideal climatic conditions.

Michele R. Warmund
Department of Horticulture
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