Pruning this late in the summer can create problems for your trees and shrubs. For some shrubs like lilac, when you cut off the tips of the stems after mid-July to shape the plant, you are removing next year's blossoms. Dead wood should be removed whenever it is noted and at the point where live and dead tissues meet. Avoid the tendency to cut back into healthy tissue when removing dead wood as this stimulates the live cells and reduces their ability to acclimate for winter.
Removing limbs later than mid-August to shape trees or shrubs - or to remove branches interfering with mowing, walking along the sidewalk, etc. - can reduce its winter hardiness. Gardeners often shear shrubs in the fall to "get them ready for winter." In the spring, they discover these plants need to be sheared again to remove all the dead stems that dried out and died during the winter. Cutting back individual long stems that give the plant a scraggly look is OK, but don't shear the whole plant. Several inches of the stems, where you made your cut to remove scraggly growth, will most likely still die back during the winter. But that dead tissue is soon hidden by the growth of the healthy non-damaged shoots.
Many shrubs here and in Denver are being transformed into lollipops and marshmallows. This results in a thick layer of leaves in the sheared zone restricting the amount of light reaching the center of the plant. Buds only form where sunlight is adequate. Shearing results in very little, if any, internal growth and it creates a dead zone inside the shrub.
A great example of this is the Pfitzer juniper you planted next to your driveway. As this plant gets larger, it becomes so large you have to cut it back in order to get into the driveway. When you cut this plant back, you suddenly realize there is no healthy tissue inside the bush and you end up with an ugly bush. This area of the plant is known as the "dead zone." The damaged portion of the plant does not repair itself, resulting in an ugly plant. If something happens to your shrub to damage the perfect shape you are trying to maintain, the dead zone will be obvious and the plant will remain that way for years. It is best to allow plants to develop their natural shape. You can still cut back the scraggly shoots, but don't shear.
Trees are similar to shrubs in that they suffer from pruning at the wrong time of year. Tissue stimulated by pruning too late in the fall can suffer from winter dehydration. Apple trees pruned too late in the fall suffer from a condition called "black-heart" disease. This happens when a live branch or portion of a branch is removed. This exposes stem or trunk tissue that dries out during the winter, resulting in the affected internal part of the tree turning black (hence the name "black heart"). "Black-heart" disease occurs on most trees as a result of later fall or winter pruning, but most trees don't respond to this damage by a color change. When "black heart" occurs, the tissue affected can no longer serve as a source of water, nutrients or energy for the continued growth of the tree. This tissue is enclosed in chemical and physical boundaries, and separated from the rest of the tree.
This condition is one reason why fruit trees pruned in late fall or early winter have a shorter life than fruit trees pruned in early spring. The same is true for other trees.
In addition to "black-heart" disease, this damaged tissue opens the tree or shrub to invasion by various disease organisms. Cytsopora canker (also known as Valsa canker) is one of the wound/stress diseases that attack trees pruned at the wrong time. Spruce, stone and pome (i.e. apple and pear) fruit trees, aspen, cottonwood, willow, and other trees are more susceptible to this fungal pathogen when stressed by pruning at the wrong time.
Dr. Curtis E. Swift is the area horticulture agent with the CSU Extension. Reach him at Curt.Swift@mesacounty.us, visit WesternSlopeGardening.org, or check out his blog at http://SwiftsGardeningBlog.blogspot.com.