The Science of Pruning
What should I prune when? is one of the most common questions I am asked.
I answer that we prune for quality — with exceptions.
In general, prune summer- and fall-flowering plants in the early spring so the plant can produce an abundance of new branches on which flowers will develop and bloom. Buddleia or butterfly bush, roses, hydrangea, crape myrtle and elaeagnus fall into this category. Early spring pruning gives these plants encouragement to produce an abundance of new stems upon which flower buds can rise, generally near the ends of the new branches.
Spring is also time to prune broadleaf evergreen plants grown primarily for their foliage. Acuba, boxwoods, cherry laurel, hollies, photinia, osmanthus and like species are generally pruned heavily in the spring and trimmed to maintain their shape during the summer.
In general, prune spring-flowering plants after they have finished blooming. So it’s not yet time to go to work on quince, forsythia, weigela, camellia, flowering crabapples, lilac, azalea, rhododendron, mountain laurel, andromeda and many more. Wait until they have finished flowering.
But there are exceptions.
Despite the fact that fruit trees bloom in the spring, fruit trees and shrubs are pruned in early spring for the purpose of reducing the number of flowering branches so that the remaining branches will produce large and attractive edible fruit.
To grasp the logic of pruning, you have to understand the role vegetative buds play in plant growth. Some plants produce an abundance of vegetative buds that can be forced into growth when branches are removed. Other plants do not. Plants that do not produce lots of vegetative buds — as well as plants unable to spontaneously grow new buds — die when pruned too severely.
Thus narrow-leaf evergreen species must be properly identified before deciding on the method of pruning. For example yews, commonly referred to as Taxus, can be pruned severely in the spring, for they will regenerate new branches from the remaining stems. However, do not excessively prune junipers, pine, fir, spruce, arborvitae, Leyland cypress, Chamaecyparis or Cunninghamia because these species are not able to generate new growth from the bare stems.