Pruning Trees and Shrubs
When to prune
A dead, diseased or injured part should be pruned out as soon as possible. Light pruning can be done anytime of the year, however avoid pruning in late summer/early fall when new growth will not harden off before freezing. The very best time to prune depends on the species, but in general runs from late fall to early spring when the plant is dormant. Do not prune plant tissue that is frozen. Maple and elm are best pruned in early summer as they tend to 'bleed' in the spring. Flowering shrubs such as lilac or honeysuckle, which bloom on last year’s buds, should be pruned after they flower. A rule of thumb is: If the shrub blooms before May 31 prune immediately after flowering - if it blooms after May 31 prune while the plant is dormant.
Pruning cuts
It is very important when pruning to make the proper cuts. Cut back to a strong lateral branch, with a smooth close cut, just beyond the branch collar. The branch collar is the swollen area where the branch joins the trunk. Cutting this way allows the tree's natural healing process to close the wound quickly. If a stub is left, it dies, permitting disease and insects to enter. Use tools that are large enough to do the job. If you have to "double-cut" with the clippers, they are not big enough or you need a saw!
How to prune
When pruning, try to preserve the natural form of the tree. Thin out weak, broken and diseased branches, narrow crotches, crossing branches and branches which are too close together. Generally, we want to leave a central "leader" to control the tree and "head" the side branches to an outward turned bud. Birch and several other species cannot be "trained" this way, and should be pruned to have several leaders.
Large branches should be removed in 3 steps. The first 2 cuts reduce the weight of the limb and help to prevent the bark from being stripped away. The last cut removes the remaining stub.
Pruning evergreens
Conifer branches should never be cut behind the area of green growth. Junipers and Arborvitae have a dead zone where no new growth takes place. Be cautious not to cut into this area or you will be left with nothing but a bare branch. Evergreens such as pine and spruce can be pruned in the spring by simply pinching the expanding candles.
Disinfect as you prune
Rubbing alcohol or household bleach diluted 1:10 with water will work as a disinfectant. Dip pruning tools into the disinfectant as you work. Always be sure to disinfect when going from one plant to another, and whenever pruning diseased plant tissue. Oil pruning tools after use as the disinfectant will corrode metal.
Pruning paint/dressing is not recommended. It actually does more harm by trapping disease organisms next to the wound.
Avoiding problems
A lot of pruning problems can be avoided by proper planting. Don’t plant under utility lines or close to sidewalks or driveways. Consider the mature size of plant materials. If a tree or shrub needs severe pruning more than every 3 years to keep the size under control, remove it and replace with one whose natural size is more appropriate.
Topping
Trees that have been topped are extremely vulnerable to internal wood rotting organisms and insect invasion. If you observe a branch of a tree, you will notice that it has a "branch collar". If the branch is removed just beyond the branch collar, the collar tissue quickly forms a protective barrier to keep decay organisms out of the heartwood. Large main trunks do not have this structure, and remain open to infection. The decay may proceed inside the tree without detection for a number of years, eventually resulting in major branches breaking or the tree coming down, unannounced. The vigorous sprouts which form at or near the cut are poorly connected to the tree. Since they are poorly connected, they are subject to unannounced separation in wind storms or under snow loads, especially after they have gotten big enough to do some damage when they come down. If the height of a tree must be reduced, drop-crotching is the approved method.
If you would like more information on pruning, email butte@uidaho.edu, or call (208) 527-8587. In addition to several publications, there is a video available to help you become an expert at pruning. For more details on pruning, go to Idaho Landscapes & Gardens - TREES, SHRUBS, & VINES.