Let’s begin this post with an economics analogy. The currency of a tree is photosynthate (sugar), the leaves are the money makers, the roots and stems are the banks. Trees use a lot of money in the spring to buy new leaves to produce more money. So in the spring they are a little economically vulnerable.
In the summer the leaves have been making good money and they are starting to put some of it into storage. The banks are also using this money to make other investments like healing wounds and creating seeds. In the fall, the tree extracts much of its assets back from the leaves and puts energy into creating buds (which contain all the cells necessary for the next growing season). It also puts energy into storage so it can survive next year’s spring shortage. In the winter the tree must use some of the money reserves to survive and uses some energy to expand its roots in soils still warm from the summer.
Based on this analogy the worst time to prune would be the spring or early summer, since you would be hitting the trees at their weakest stage. A good time to prune could be the winter when trees are not actively growing, or in the summer, when they can quickly and easily respond and heal the pruning cuts. Fall pruning can shock a tree into unnecessary growth or create wounds that will not be able to heal very quickly.
This analogy is an oversimplification of some very complicated science, and a lot of nuances are ignored, but it helps get the idea across.
Pruning fruit trees during the dormant season will therefore maximize the tree’s ability to adapt and adjust to the “injuries” you’ve created. Come spring, they will begin to ‘heal’ those wounds and they will focus their resources and stored energies on existing buds. Sometimes heavy pruning can stimulate dormant buds near the pruning cuts or along the pruned branch. These buds release “epicormic shoots” that can be seen as vertical sprouts along the branch (often called “water sprouts”). For this reason a second pruning later in the season can be beneficial to direct the tree’s resources away from these unwanted vertical shoots and into desired branches. In a sense, pruning in the winter allows the tree to begin its growing season without missing a beat. Pruning in the summer can be good if you want to slow the tree down.
A big reason to prune in the winter as opposed to the summer is to limit the spread of some of the aforementioned diseases. Fungal and bacterial pathogens thrive in moist, warm conditions. Therefore, don’t go out and do all of your pruning on a warm rainy day. Cold, dry conditions are a good time to prune fruit trees.
Pruning in the winter is also a good idea because the scaffolding of the branches and the fruit spurs are more obvious this time of year. Creating appropriate and healthy scaffolding is the basis of a good structural pruning. “Fruit spurs” are the buds along the branch that will become flowers, and then fruit, as opposed to leaves and branches. Knowing how to identify the fruit spurs is helpful so you can thin the fruit and again, direct the tree’s resources towards less fruit, thereby making the existing fruit more desirable.
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