Introduction
There is an idea that gardeners have no business pruning their own trees. I don't believe it. That idea means that as homeowners our main concern is to write the cheques and keep our hands off.
Pruning your own trees can be a very rewarding experience. As the years pass, and you and the trees grow, you'll find few things ground you as much to place; more so, if you have participated in their care.
The Calgary environment is a special and harsh place. We live at the extreme northwest corner of the Great Plains. Our proximity to the mountains ensures that our weather gets shook up regularly. There are the chinooks, temperature fluctuations and drying winds to test the mettle of any tree. Consider that Calgary has only four native trees: balsam poplars in the riparian areas, trembling aspens on the sunny hillsides, and white spruce in the river bottoms and north-facing slopes where they are joined by Douglas firs.
Be wary of information that doesn't see Calgary as a highly individualistic area when seen on the broad map of North America.
Pruning Theory
Pruning theory can be as complicated or as common sense as you make it. Realize at the outset that your tree is probably not that bad. The most important aspect of growing trees is leaving them alone, doing only what is needed to keep them healthy. Pruning that greatly reduces leaf mass is injurious to the tree's system. Before you touch your tree, see it for what it is. Learn its name and be able to identify the same species elsewhere; parks and the University campus are good for this. By looking at others, you will learn what is normal, and what is a variation. After inspecting several of the species, look at your tree again. Focus on the four D's: diseased, dead, damaged and density. If you are convinced that it needs some pruning, then you will need four tools: a good pair of secateurs, a handsaw with a Japanese tooth pattern that cuts on the pull stroke; a sturdy four-foot wooden ladder; and finally, a pole pruner for reaching. I use a holster and scabbard on my belt for the cutting tools, to keep them close when I need my hands free.
There are two types of pruning cuts that arborists perform: first, a branch collar cut that removes an entire branch; second, a branch shortening cut that removes the leader or branch tip. If possible, make all of your pruning cuts branch collar cuts. The collar plays a major role in tree defence and by pruning there, you do the tree the least harm. The branch shortening cut is done only when the removal of the branch would be an energy loss, or if its removal would create a visual problem, a hole in the shape.
Looking at the Diagram
Look in the crotch of any branch and you will notice a small ridge of bark tissue. This is the branch bark ridge (BBR). It is the result of the growth between the trunk and a side branch. The BBR defines the top edge of the collar. The sides of the collar are defined by the diameter change between the branch and trunk. The branch, where it starts, is slightly smaller than its collar. When one starts identifying where collars are, looking and feeling are both necessary.
For a shaping cut, your focus shifts to the other side of the BBR. Leave a small margin above the BBR and cut back along a line parallel to the lateral branch you are leaving as leader. When pruning large branches, you should first stub cut the branch, that is, remove most of the branch and its weight. By doing this, you will reduce the chance of tearing out the bark below the collar. Undercut the branch first, then cut through from the top. It is much better to be a little farther out than too close, which would create a flush cut. Flush cuts are ones that remove some or all of the collar and cut into the trunk. These cuts create a voluminous injury in the trunk. They are the worst pruning cuts possible.
Disease
Removing diseased branches to stop reinfection is no different from either the collar cut or the branch-shortening cut. The two most common diseases you will have to deal with are fire blight and black knot.
Fire Blight
Fire blight is caused by bacteria that live on starch. It can be completely dealt with by pruning. The tree's first contact with the disease (usually delivered by insects) is on its flowers and branch tips. If you have any variety of apple, mountain ash or pear, you should inspect your trees several times during the growing season when the blight is active. If the tree shows any brown dead leaves at branch tips, there is a good chance your tree has blight. Never cut through a blighted section of a branch, infecting your tools. If you do, spray or dip them in 10% solution of bleach. I carry a small spray bottle with me in the tree. If you miss these first easily dealt with symptoms, bad things can happen. The disease progresses along the branch, creating a bacterial ooze, which drips onto the lower branches and trunks, infecting them. The ooze is easily moved from tree to tree by birds and insects. All varieties of apple are subject to fire blight, but some are highly resistant. I often see old dead lesions on apples where blight got a start and then was contained. If apple trees had no natural ability to deal with the disease, there would be no apple trees left. In some varieties, the disease has a near free run, for example columnar apples. In others, for example Dolgo, it just sits. The trick is to inspect and prune often.
Black Knot
Black knot affects only cherry trees. In Calgary, its most common host (some 80%) is the Mayday. To a lesser degree the Schubert cherry is affected. I have never seen black knot on an Amur cherry, though it is rampant in wild chokecherries. The chokecherry is our major source of black knot spores. Fish Creek and other wild areas abound with them.
Black knot is a fungus that can grow to some size, wrapping and growing along a branch, an elongated black swelling. When mature it releases many spores that can reinfect or be carried to other trees. The cure is removal by pruning. The best time to find all your black knots is after leaf drop. Black knot will kill any branch it persists on. Sometimes a knot will form right on the trunk or main crotch of a young tree. Rather than remove the tree, I have sawed off the knot, leaving a wound but keeping the tree. Like all those in nature who feed off of others, black knot occasionally loses due to a strong defense system. I have seen old black knot sections dead and completely surrounded by healthy tissue.
Density
Bad practices die slowly in the tree care industry. We have nearly got past topping; this is good. The next bad practice is over-thinning, an unnatural reduction of density. Each species has a look, a pattern, that you should understand before you touch the tree. One of the old rules that has helped create the over-thinning problem concerns crossing branches. Outdated practice recommends removing all crossing branches or branches that touch another. On a Mayday, this would leave you with a scarecrow that would either sucker profusely or start to die back. Young branches that grow from one side of a tree into another should be removed. Old branches that cross should stay. The energy gained from the leaves of a crossing branch far outweighs the possible negative effect of an infection site. Trees know what they are doing. Think of the whole system, roots, trunk, branches and leaves as a unit. A tree is more than our names for its parts. A tree's potential long life and health are about conserving energy. This is accomplished by a full crown and a large reserve of healthy tissue in trunks and branches where starch can be stored and retrieved as needed. Every pruning cut takes away from that reserve.
Conclusion
The study of pruning is full of opinion, fact and fallacy. People often cling dogmatically to rules they don't understand and can't explain. If you want to prune, do it when you're ready. Make your move, then wait for the tree's reaction. Let one growing season go by. Are the wounds closing up nicely? Did the tree sucker from too much leaf loss? Trees are incredibly tough and resilient. Witness the survival of many topped trees. Years later, most people can't tell it happened. Twenty-five years ago pruning jobs that removed a third of the branches, made flush cuts, used pruning paint and topped followed the rules then in vogue. So go ahead, do a little maintenance and enjoy your trees all the more. Prune your trees with respect and understanding and you can't possibly hurt your trees.
Two books by Dr. Alex Shigo that shook up the tree service industry are A New Tree Biology and Tree Pruning, a World Wide Photo Guide. The branch collar and many other critical aspects of a tree's defense system were first explained by Dr. Shigo.
