Bunkey was sitting at the kitchen table one morning watching the birds when he noticed that some of his trees were looking a bit scraggly. One had a broken branch he hadn’t noticed before and a few had water sprouts he missed last spring. Another had some crossing branches that should have come off.
"Well, it’s nice out so I’ll go out and hack those bad branches off now, so I don’t have to do it in the spring," he says to himself. Don’t do it! He can cut off any small or diseased branches, very light pruning, but the more extensive job should be done in March or April, if the threat of severe cold has passed. After all, this is Minnesota where the weather can turn on a dime and give you eight cents change. Yes, it would be nice if you could prune trees whenever you have the time, but there is a good scientific reason for the late winter date.
During a year, a tree goes through five major phenological growing periods where the tree’s stored energy fluctuates. From February through March, the tree is just coming out of dormancy. It is just starting to wake up and grow a little, relying on reserve energy from the last year. In the next period, April, the tree is still relying on last year’s energy to start bud break and leaf formation. It is at its lowest energy level of the growing season. Bugs and pathogens are also becoming active now.
From May through July, the tree is actively growing. All of its expanding height and width occur within the six to eight weeks after full leaf expansion. August to September the tree starts to store energy for next year. That is why you should be sure they have adequate amounts of water during dry periods. From October to January, the tree is just loafing, probably hating the cold and waiting for spring just like we are.
Now you should be able to see why pruning should not be done during the active part of the growing season when the tree has little energy to close wounds. Pruning in the wrong season can even stunt your tree. Reducing leaf area reduces the trees’ ability to photosynthesize food and can shorten its life. The exception to this rule is; birches and maples should be pruned when fully leafed out as they "bleed” in the spring.
This is why you should prune just before new shoots emerge. The wound will heal quickly, there are few insects or disease spores active and the tree has not leafed out so you can see what you are doing.
Oaks and elms should never be pruned during growing season as the chance for Dutch elm disease or oak wilt getting a foothold in fresh wounds is much greater than.
Fresh wounds made in the fall are more prone to decay as they don’t heal rapidly as in the early spring. There are exceptions to this rule. Birches and maples should be pruned when they are fully leafed out, unless you are collecting sap. They “bleed” sap if pruned early.
Now about wound dressing; don’t! It actually slows healing. It also may harbor disease organisms. Again, there are exceptions to this rule. If you must trim an oak tree at the wrong time of the year because of storm damage, have an opened can of latex paint handy. In other words, paint the wound immediately. This is thought to discourage sap beetles that may be the vector for oak wilt fungus .
Now is the time to be thinking about Christmas trees, not trimming lawn trees.
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