Pruning trees and shrubs is key to maintaining proper form and can also act as a preventative measure for the control and spread of disease. Many problems may be avoided by pruning properly in the formative years of a tree or shrub. Proper pruning at a young age will force the tree or shrub to form a solid structure. When planting a tree, you’ll want to make sure you only remove dead, diseased or broken branches. It’s best to start to train or form a tree during its first dormant season; this ensures you won’t affect its flowering stage.
There are two basic tree pruning cuts: The heading cut, which cuts back to an intermediate point, and the thinning cut, which cuts back to a point of origin.
Removing a portion of a growing stem down to a set of desirable buds or side-branching stems is commonly performed on well-trained (diligently pruned) plants for a variety of reasons; to stimulate growth of flowers, fruit or branches, as a preventative measure to wind and snow damage on long stems and branches, and to encourage growth of the stems in a desirable direction. This is also known as heading back.
Thinning – A more drastic form of pruning, a thinning cut is the removal of an entire shoot, limb or branch at its point of origin. This is employed to revitalize a plant by removing over-mature, weak, problematic and excessive growths. When performed correctly, thinning encourages the formation of new growth that will more readily bear fruit and flowers. This is a common technique in “opening-up” the branching of neglected trees or renewing shrubs with multiple branches.
Topping – Topping is a very severe form of pruning which involves removing all branches and growths down to a few large branches or to the trunk of the tree. It is most often used on very young trees.
Raising – This technique removes the lower branches from a tree in order to provide clearance for buildings, vehicles, pedestrians and vistas.
Reduction – This method will reduce the size of a tree, often for clearance for utility lines. Reducing the height or spread of a tree is best accomplished by pruning back the leaders and branch terminals to lateral branches that are large enough to assume the terminal roles (at least one-third the diameter of the cut stem). Compared to topping, reduction helps maintain the form and structural integrity of the tree.
Deadheading is the act of removing spent flowers or flowerheads for aesthetics, to prolong bloom, promote rebloom or prevent seeding.
Young shrubs do not require as delicate and precise pruning as trees, but care and consideration should still be taken. Shrubs, like most other plants, come in a variety of sizes and pruning should be approached depending on individual growth habits. They can have mounding (azalea), cane (forsythia) or tree-like (rhododendron) habits and understanding these will also help you understand how to prune.
As with trees, the two main types of cuts are heading and thinning cuts. Heading cuts serve to stimulate the growth of buds closest to where the cut is made, while thinning cuts remove branches at their point of origin and therefore reduce shrub density. It is extremely important that cuts are made correctly for proper effect. Heading cuts should be made ¼ inch above the bud, sloping down and away from it. If cut too close or steep, the bud may die. Thinning cuts should be made just above parent or side branches and roughly parallel to them.
Maintenance pruning – Deciduous shrubs require this type of pruning to keep them healthy and in scale with their surroundings. Maintenance pruning should begin at the time of planting, or for rejuvenation of older shrubs.
It is crucial to remove any dead, diseased or broken branches promptly. For shrubs with cane growing habits, first remove the tallest canes by cutting them out near ground level. Then, thin out any canes crowding the center or growing in an undesirable direction. For mound-type shrubs, prune only the longest branches. Make thinning cuts well inside the shrub mass where they won’t be noticeable. This method reduces the size of the shrubs without sacrificing shape.
Lastly, tree-like shrubs can tolerate removal of ⅛ to ¼ of their branch length. Wait until the very end of pruning to make any heading cuts on them. Keep the crown open and maximize light penetration by using careful thinning cuts.
Rejuvenation pruning – The following should be consider when pruning older shrubs:
- Select an appropriate species. Not all shrubs respond well to drastic pruning
- Observe timing. The appropriate time for this type of pruning is early spring just before bud break
- Give extra care (fertilization, watering, pest control) to heavily-pruned shrubs
- Consider the shrub’s new appearance. What will the impact be on the landscape?
There are two basic techniques for rejuvenation pruning. The first and quickest technique involves complete removal of the entire plant 6-10 inches above the ground. Remove half of the new canes that develop by mid-summer and head back some of the remaining canes. When using a heading cut, be sure to prune to outward-pointing buds so that the inner portion does not become too dense. Shrubs that tolerate extensive rejuvenation are: abelia, dogwood, honeysuckle, hydrangea, lilac, mallow, rose-of-Sharon, spirea, and St. John’s wort.
The second technique removes growth more gradually. The first year, remove one-third of the oldest, most unproductive branches. The next year, remove ½ of the old, lingering stems. Finally, in the third year, prune out the remainder of the old branches. New, productive stems should quickly replace the old wood. This method takes longer to complete, but the shrub stays more attractive throughout the rejuvenation period.