CORVALLIS, Ore. - You can prune an apple tree any time of the year without hurting it, but late winter, just before spring, is probably best. The worst of the cold weather is past, so you won't be subjecting the fresh cuts to severe icing, but you'll still be able to influence the tree's spring growth.
There are several main objectives to pruning an apple tree, says Pat Patterson, Oregon State University Extension Service Master Gardener:
- Controlling the height of the tree, so that most of the fruit doesn't grow out of reach;
- Developing good limb structure for strength, fruit production, and the general health of the tree;
- Encouraging a plentiful supply of new limbs, which will begin to bear fruit their second year; and
- Ridding the tree of damaged or diseased growth.
The overall size of the tree depends primarily on its rootstock and innate vigor. Most apple trees are grafted onto dwarf or semi-dwarf rootstock. (Take care when you plant a new apple tree not to bury the graft, where the fruiting stock joins the rootstock. This will ensure that the fruiting stock will not begin to produce its own roots and the tree will keep its dwarf or semi-dwarf height.) Even so, you'll want to monitor the height of your tree to be sure it doesn't outgrow the spot you've picked for it. Once it's as high as you want it to be, prune the central "leader," the main upright limb, back to a lateral branch. Then keep monitoring the height year by year.
"Don't expect a new young tree to start bearing well until probably its fourth or fifth year," said Patterson. "In the long run, the tree will do better to put its energy into root and limb growth rather than fruit for those first few years. So concentrate your pruning to produce a strong tree during that period."
Inspect your tree for limbs that branch from the central leader either too sharply upward, forming an acute angle, or at too wide an angle. Acute angles tend to trap bark as they grow and can lead to splitting later on. Branches that grow at too great an angle from the vertical tend to be weaker. They also encourage "water sprouts," the unproductive upright shoots that need to be pruned off mid-summer every year. The ideal angle between the central leader and lateral branches is about 60 degrees.
In general, encourage branches to grow toward the outside of the tree and eliminate those that grow toward the center or cross other branches. You want air and light to penetrate the foliage to the center of the tree as much as possible.
"Different kinds of apple trees have different ways of setting fruit buds," said Patterson. "Most modern apples are spur-bearing. Many older varieties are tip-bearing. This is obviously very important for how we prune the tree so as not to cut off the fruiting wood. If you're in doubt, as long as you know the name of your tree you can ask at your local nursery or look it up in a good garden book or on the Internet."
Once your tree has matured and begins to produce fruit, expect new branches to bear their best for several years (perhaps three to five years) and then taper off. You'll want to prune off older branches that have begun to produce less in order to encourage new ones. This practice will help you have a more-or-less steady crop over a period of years.
Summer is a good time to remove older branches, according to Patterson, because it is then obvious which branches are producing best and which should be pruned. Summer pruning also allows you to get rid of branches that are showing signs of damage or disease as soon as you spot them.
Beyond these basics (which also apply to other similar fruit trees, for instance pears) there are many fine points to pruning a fruit tree. For instance, how far from the central leader should you cut a lateral branch? At what angle? Should you shorten branches or always take them back to the central trunk? Are water sprouts ever good to keep?
The OSU Extension Service has several publications on planting, growing and caring for home fruit trees: