Most growers prune apple trees in mid to late winter during the tree's dormant period. In winter the tree will be less exposed to insect damage and infection by fungal spores. Over the winter the wound will harden and seal off. During the growing season of spring and summer surrounding tissue will overgrown the scar. For mature fruit-bearing apple trees some thinning and trimming will be needed when the tree is in leaf, but major work should be saved for the off-season.
Fruit bearing apple trees produce more and better quality apples if trained to a shape that distributes sunlight evenly through the levels of the tree. Horizontal branches bear more fruit than the more vigorous vertical or sucker shoots. An apple tree's natural growth habit yields several upright trunks with high branches. The natural shape does not encourage maximum yield or easy harvesting. Winter pruning encourages a fertile shape for the growing season.
The leader or central shoot of a young apple tree should be clipped off in late winter. In the home garden a new whip or year old apple should be headed back at about 30 inches high the first winter. This encourages the side branches to develop. A low spreading shape makes caring for the tree simpler, giving access to leaf surfaces and bark when spraying becomes necessary. As the tree ages, continue to cut back the vertical shoots and favor the fruiting branches. Allow only about 2 feet of vertical growth per year.
An old rule of thumb for pruning fruit trees was to cut out enough branches so a bird could fly through. This open airy structure allows sunlight to reach the different layers of branches. Without an even distribution of sunlight, fruit ripens unevenly and fungal diseases become more likely. Pruning out about one third of the canopy each year invigorates the tree and increases yield. If the twigs or fruiting spurs at the ends of branches grow too thickly, trim these out as well. Remove any dead spurs and open up the shape of the branch.
In summer sucker shoots or water sprouts--vigorous vertical shoots--may draw away much of the growth energy of the tree. On branches head sucker shoots back by clipping them off above the first fruit cluster or back to the main branch. Suckers on roots or the main trunk should be clipped off at the base. Any remaining water sprout growth on branches should be completely trimmed out during the winter dormancy.
Old untended apple trees often stop bearing fruit or bear large crops of very poor quality fruit. Pruning these old trees back to an efficient shape can take several years of careful planning and tending. Prune too much wood away in a season and the tree may be damaged by infection or sunburn. Open up the canopy by degrees, encouraging side growth and removing shaded branches. Clip off ends of heavy drooping branches. Correct pruning should invigorate the old tree even though the final shape will not be ideal.