While trees are dormant, it’s prime time to consider pruning to get them ready for the growing season. Not every plant requires pruning; however, pruning can help improve the structure and health of trees. Pruning is both an art and a science.
There are two different types of pruning. The first is called crown cleaning, or is commonly referred to as the three D’s — removal of dead, diseased and dying portions of the tree canopy. The second is what we’re going to focus on here: crown thinning.
Crown thinning is just what it sounds like: You prune out portions of the tree to open up the canopy. Crown thinning improves airflow, increases light penetration, removes vigorous upright water sprouts and improves overall tree health.
As we have seen in previous springs, foliar diseases such as anthracnose and apple scab are rampant. One factor that contributes to these foliar diseases is high relative humidity. When leaves stay wet for extended periods of time, it creates a good microclimate for fungal infection. By thinning the tree canopy and allowing for more airflow, you can reduce relative humidity, which allows leaf surfaces to dry out more quickly. Understand that just thinning the canopy will not prevent foliar diseases. But it will help reduce and possibly shorten the disease incidence.
What thinning accomplishes
Why would you want to improve light penetration? One reason is for fruit ripening. For example, peaches require quite a bit of light to ripen and develop that nice reddish-orange color, which indicates the fruit is ripe. Thinning also provides for growth of interior branches and leaves by increasing photosynthesis.
Water sprouts are those fast-growing, vigorous shoots that usually result from excessive pruning and rob the tree of energy. They are vegetative and do not contribute to fruit formation in fruit trees. Within a few years, they can reach the size of the limb they are growing from, giving the tree poor structure. In addition to not bearing fruit, water sprouts are a real headache when you go to pick fruit. If you’ve ever tried to pick from a tree that has a lot of water sprouts, then you know what I am talking about.
However, before you get too aggressive in removing water sprouts, remember, all things in moderation. Do not remove all of them at once, as this will just cause more to sprout the next growing season. Also, removing all that vegetation from the center of the tree may cause sun scald and blistering of the bark on hot days. Consider removing a portion of the sprouts over several growing seasons.
Another reason to thin the canopy is to provide good tree structure. Limbs that are close together or two limbs growing parallel to each other do not provide for good tree structure. Both limbs are not needed, so removing one of them will improve the longevity and architecture of the tree. Decide which limbs will be your scaffold branches, and then remove unwanted branches in subsequent years.
As I mentioned at the beginning, pruning is both a science and an art. Before you make that first cut, step back and look at the tree. Ask yourself, “Why am I pruning this tree?” Study its shape and form, and be knowledgeable of how the tree will respond to pruning.
Keep in mind, you are working with a living organism. Follow the 25% rule: Do not remove any more than 25% of the tree canopy at any one time. Excessive pruning will put the tree under stress, ruin its shape, and could lead to tree decline and death. If possible, take a class on pruning and practice. For further details on pruning woody plants, consult your county Extension office or local certified arborist.
Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at email@example.com.