Craig Light pruning to remove a few small branches of trees can be done at any time of year, but more extensive pruning should be done in late winter to early spring. There are three advantages to pruning at this time of the year. First, wound closure is most rapid if it's done just prior to when new shoots emerge. Second, there are few insects and disease spores to infest pruning cuts. And third, deciduous trees have dropped their leaves, so it's easier to see what you're pruning. When removing any branch, large or small, avoid stubs and cuts close to the trunk. As a guideline, look for a bark ridge located in the branch crotch. You also will find that most branches have a slight swelling or "collar" at their base. Avoid cutting into this because that will destroy a natural protection or boundary in the tree.
Trees such as elm, maple, walnut and birch are "bleeders" when pruned in late winter to early spring. This oozing sap is annoying when it drips on cars and sidewalks. There is disagreement as to whether the oozing is harmful to trees. It is thought that oozing might interfere with the closure of pruning cuts. Oozing is reduced if these tree species are pruned in the fall. However, fall is probably the least desirable time of year to prune most trees. Pruning cuts close more slowly in the fall than in any other season.
Training a young tree
It is important to leave as much healthy growth as possible the first year a tree is planted to provide foliage for food production. This is needed for root establishment. In the second year, select the branches that are well-spaced up and down the plant and leave these. Remove all other branches, particularly when they interfere with each other, arise from the trunk close to the same point, or have acute, upright crotches. These last tend to develop "included bark" as the tree grows. Such branches will be weak and fail under heavy fruit loads or snows. In subsequent years, select additional strong branches and remove weak and interfering ones. It also may be necessary to remove some lower branches that were left earlier.
Pruning older trees
Avoid pruning any more than necessary. Some pruning books urge "topping" older trees to stimulate young, productive wood. This practice, however, not only leads to the development of weak shoots, but also invariably opens a tree to numerous disease and wood-rotting organisms. Canker diseases such as Cytospora, common in peaches, and bacterial diseases, such as fire blight in apple and pear, are perpetuated through unnecessary and improper pruning.
Occasional thinning of branches (never the tops or branch tips) may be necessary. Undercut large branches to prevent bark-stripping when the branch falls. Then remove most of the branch weight with a cut outside of the undercut. Make a third cut outside of the branch bark ridge and collar to remove the stub. Do not make flush cuts, as has been incorrectly described for many years. Dr. Alex Shigo of the U.S. Forest Service discovered that flush cuts remove a natural boundary into the tree, thus opening the tree to infection.
It is tempting to apply a wound dressing to a pruning cut. This age-old practice, however, has been shown by several research scientists to be of no benefit to the tree and can even harbor disease organisms. Wound dressings are sometimes desirable for aesthetics, to hide or camouflage the exposed wound. If this is the case, use the type of material in aerosol cans available at Garden centers. Apply a thin coat. It should barely coat the wound, changing its color but not forming a thick, black film.
For more information, see the following Colorado State Extension fact sheet(s) at www.ext.colostate.edu:
• Training and Pruning Fruit Trees
• Pruning Mature Shade Trees
Or contact Elisa at the CSU Moffat County Extension Office, 539 Barclay, 824-9180.