People worry about planting, fertilizing, pruning and picking the fruit from an apple tree, but tend not to think about the tree's pollination process. We tend to assume that the bees will automatically spread the pollen and apples will grow, if we think about pollination at all. However, if you understand the science behind the growth of an apple, including its need for pollination, you will grow a better crop of apples.
Most apples are self-sterile, which means they need another tree to pollinate their blossoms. This is a natural protection factor in the sense that it keeps disease from wiping out trees since the DNA of both trees--at least one of which might be resistant to certain diseases--are combined to form the fruit. Apple species such as Lodi, liberty, empire, winesap, jonathan, jonagold, gala, golden delicious, Rome and Granny Smith are a few trees that growers consider self fruitful, but still need to have pollinating trees planted close by for better fruit production.
Time of Bloom
Apple trees will blossom at different times of the summer. Some start early, like the honeycrisp, empire, liberty and Manchurian crabapple, while others, like the gold rush or Rome blossom several weeks later. If the blooms do not open at approximately the same time, the bees will not be able to transfer the pollen. Plant at least two varieties of apples that bloom at similar times. The University of Missouri provides a nice chart of apples with their blooming times. Find a link in the Reference section.
Bees actively pollinate by buzzing from flower to flower. If the two trees you planted grow too far apart from each other, the pollen simply will be dropped before the bee ever gets to the neighboring tree. If you plant dwarf trees about 20 feet apart from each other or standard trees 50 feet apart, your pollination should succeed.
Within the apple varieties there are some trees that will not cross-pollinate each other. Check the the University of Missouri Extension chart in the Reference section to see if the varieties you wish to plant are compatible with each other. Another option is to plant a Manchurian crabapple for early bloomers, and the snowdrift crabapple for late bloomers. A trick some apple growers use is to hang buckets of freshly cut blossoming apple or crabapple branches in the target apple tree. This will bring pollen to the tree when the neighboring trees are set too far apart.
If you think about your apple blossoms the way a bee might, you might see your yard a little differently. Bright yellow dandelions look pretty attractive to the wandering honeybee, and by the time he has finished visiting the 100-plus blossoms, there will be no apple pollen left on his legs to distribute. Make sure you mow down any dandelions around the trees and keep them mowed until the apple petals have dropped.