In order to have successful pollination, it is necessary to have two different varieties of apple trees. Most apple varieties are self-unfruitful, which means their blossoms must be fertilized with the pollen of a separate variety in order to achieve good fruit set. A few varieties, including 'Rome Beauty' and 'Newtown', are considered self-fruitful, meaning their blossoms can be fertilized with their own pollen, but even these apples produce more fruit if they are cross-pollinated by another variety. It's important to choose varieties that have compatible pollen and bloom times. The Raintree Nursery catalog and Web site have a very helpful pollination chart that details which apple varieties are compatible.
Apples also need pollinators—certain wasps, flies, and bees—to transfer pollen from one variety to the other. The apple trees must be planted within 100 feet of each other in order to help ensure that the pollinators visit both trees.
If you have only one apple tree in your yard or incompatible varieties, all is not lost. Crabapple pollen fertilizes apple blossoms. So if you have a crabapple in the vicinity that blooms concurrently with your apple tree, you're in business. Grafting a branch of a compatible variety onto your existing tree is another option, though I recommend you hire an arborist to perform this job. You can also use an old, very effective orchardist trick, says Matthew Rogoyski, Ph.D., a horticulturist at Colorado State University. "Put a bouquet of crabapple branches in bloom in a 5-gallon bucket of water and place it inside the canopy of the tree," says Dr. Rogoyski. "Then bees can visit the crabapple blossoms and transfer the pollen to the apple blossoms."