Pruning Trees

Pruning Trees

Author(s): Graham Herbst

Pruning is one of the most important tree maintenance practices.  Over the years, the way in which we, as foresters, have approached pruning has changed dramatically.  Today pruning is a science that, if not done properly, can be very damaging to a tree. Here are some tips that may be of help to you before you take the saw to the tree. 

What to Prune

First, large tree pruning can be very dangerous work.  Even the most highly skilled arborists have been seriously injured while pruning a large tree.

Most pruning should be confined to removal of broken, rubbing, damaged or dead branches. Sometimes pruning is necessary to elevate tree branches over a sidewalk, street, roof, or other structure.  It is generally not necessary to thin a tree out so that more air passes through the crown.  This practice is sometimes known as "lion tailing" and can leave the tree open to stress during the hot months of summer.

If you are cutting live branches, never remove more than about 20% of the total leaf area of the crown.  If trees need to be elevated (removal of lower branches), the process can be done in stages over several years if a large amount of work needs to be done.

Making Pruning Cuts

Pruning cuts should be made so that only branch wood is removed and the trunk or supporting stem is not injured.  If only branch wood is removed, the wound is smaller, the tree will be able to seal the wound more effectively, and the chance of problems with wood decay will be greatly reduced.

To locate the proper place to make a pruning cut, look for the "branch bark ridge" on the
upper surface of the union of the branch with the supporting stem.  This is a line of bark that has been pushed up as the branch and supporting stem have grown.  Some branch unions will not have this if they did not form properly.  Instead, they will have the branch simply pressing into the supporting stem, forming a sharp V-shaped union.  At the base of the branch, and mostly on the underneath side, look for the "branch collar," which is a slightly swollen area of stem tissue that wraps around the base of the branch.  A proper pruning cut begins just outside the branch bark ridge and angles down and slightly away from the stem, avoiding injury to the branch collar.

Do not make flush cuts that remove the branch collar.  Wounds that are created by flush cuts cause substantially more injury to the tree than wounds left by proper pruning.

Diagram of pruning technique
For larger branches, follow this three-step process to eliminate additional damage to the tree. (Courtesy: Arbor Day Foundation)  

When Should I Prune?

It has often been said that pruning can be done when the saw is sharp.  This is not too far from the truth.  Winter pruning will provide an entire growing season after the cut for the tree to begin the sealing process.  There are two times of the year when pruning should be curtailed.  The first is during the active spring growing flush, usually April and May.  During this period the bark is very tender and can be torn easily.  This may result in a much larger wound for the tree to seal over.  The second time is during the fall coloration period.  It is at this time that the chances of serious decay are at their height.  If you live in an area where oak wilt is a concern, pruning of oaks should be restricted from April through June.

Pruning "Dont's"

Dozens of sprouts emerge from a topped tree.
Topping a tree is not just unsightly, it can open it up to disease and pests. Additionally, trees can become more hazardous over time. This leaves the property owner liable for any damages.

Never top trees. Topping creates serious hazards and dramatically shortens the life of a tree. Never use paint or wound dressing to cover wounds. These materials do not help the tree and actually interfere with the tree's wound healing process.

Pruning Storm Damaged Trees

The only pruning that should be done immediately following a storm is removing broken branches. Leave the fine pruning and finishing cuts until later. All pruning cuts will dry out to some degree if made during the winter. Dieback of the inner bark around a pruning cut can be minimized if final pruning is left until just before the tree begins to grow in the spring.

Safety is the first consideration in removing branches from storm-damaged trees. Remove all loose branches as soon as possible to eliminate the potential for injury or damage if they fall. Next, remove cracked or broken branches. Branches that did not break under the weight of snow, ice or some other damage, but are bent, may have internal cracks or other hidden damage, especially if the branches have not returned to their upright position. These branches may become hazards in the future and should be considered for removal. A branch (or trunk) that was partially stripped of its bark when an attached branch pulled away should be removed if more than a third of the original circumference is lost. These branches are structurally weak and may become serious hazards if they are allowed to remain and gain weight. Branches that have pulled away from the trunk should be removed at the bottom of the split. Avoid causing any additional damage to the trunk. Remove loose bark, but do not cut into bark that is living and still attached.

Pruning cuts should be made so that only branch wood is removed and the trunk or supporting stem is not injured. If only branch wood is removed, the wound is smaller, the tree will be able to seal the wound more effectively and the chance of problems with wood decay will be greatly reduced. To locate the proper place to make a pruning cut, look for the “branch bark ridge” on the upper surface of the union of the branch with the supporting stem.

This is a line of bark that was pushed up as the branch and supporting stem grew. Some branch unions will not have this if they didn’t form properly. Instead, the branch will be pressing into the supporting stem, forming a sharp V-shaped union. At the base of the branch, and mostly underneath, look also for the “branch collar” which is a slightly swollen area of stem tissue that wraps around the base of the branch. A proper pruning cut begins just outside the branch bark ridge and angles down and slightly away from the stem, avoiding injury to the branch collar. Do not make flush cuts that remove the branch collar. Wounds created by flush cuts will cause substantially more injury to the tree than wounds left by proper pruning. Branches should be pruned using a series of three cuts: two to remove the weight of the branch (first under then over the branch), then the final pruning cut.