What the experts say.
Bunkey always seems at a loss as to when he should prune various trees. There is scientific disagreement as to when maples and birches are pruned. Some say pruning in the spring when the sap drips is harmful. Other swear that the sap reduces the colonization of decay organisms. Here is what expert Patrick Weicherding, an Extension educator, says.
You can remove any small damaged, diseased or dead branches at any time. More extensive pruning should be done in late winter or early spring; in March or April once the threat of severe weather has passed. That is except apple trees. The time to prune them is February. Now that we have all the exceptions out of the way, let’s look at the science behind this.
In a one year cycle, a tree has five major phonological periods. The tree’s stored energy fluctuates as a result of these periods. From February through March, the tree is coming out of dormancy. Growth is starting but very little photosynthesis is taking place. At this point, the tree is relying on reserve energy from the previous year. In April, bud break and leaf formation are the major growth activities. Photosynthesis is still minimal and the tree is still using energy from its reserves. This is the tree’s lowest energy level of the growing season. To make it worse, insects and pathogens are also becoming active. From May through July, photosynthesis increases dramatically. The new leaves begin to produce more energy most of which is used up in the early period to make leaves. All of the trees growth occurs during the six to eight weeks of full leaf expansion. By August and into September, the tree begins energy storage for the next year. Most of the seasonal growth is done and the tree enters dormancy where it just loafs along until next February.
This should make it obvious that pruning during the active growing season, when the tree has very little to no energy reserve, is a bad time to be cutting off branches. It can change the growth of the tree. Removing living foliage by pruning affects the tree’s physiology and future growth. By removing leaves you reduce the tree’s overall photosynthetic capacity. This can affect just the pruned section or the whole tree. Pruning during the growing season will have a lasting effect on the tree and can reduce its longevity.
The advantages of pruning during the dormancy period.
Wound closure is most rapid if it’s done just before new shoot arrive. Too early and the wounds can crack and dry out delaying healing. There aren’t any bugs or active diseases floating around in the winter. With no leaves on the tree, you can see the shape and where it needs pruning. Now, as to the old husband’s tale that you need to cover a wound in a tree — it’s bunk! It simply delays healing and the dressing can actually harbor disease organisms. The one exception is to prevent oak wilt if you have a wound in an oak tree during the growing season. It is used to discourage sap beetles that are a vector of the oak wilt fungus. These little stinkers can detect oak sap a ½ mile away.
Now is the time to get those pruners and saws nice and clean and sharp. And maybe, set a ladder beside the apple tree that needs pruning when the snow drifts are knee high to LeBron James the basketball player.
Bev Johnson is a Master Gardener with the University of Minnesota Extension. Her column appears in the Weekend Edition.