Bay Weekly

The Science of Pruning

By Dr. Francis Gouin

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.

When to
Repot a Cactus

  We have a cactus that we replanted about a year and a half ago. It has prospered, probably in part because when we repotted we used soil designated for cactus that included nutrients. It was about one foot high above the soil before repotting. Now it is two feet high. It appears to be healthy, even robust. We are debating whether to repot it because of its growth. But we do not want to do so if it is unnecessary.
    A couple of its stalks or branches have some discoloration at their base as they go into the soil. It sort of looks like scale but is pale brown and is quite dry compared to the beautiful green color of the rest of the plant. Should we be concerned?
   –Andrew H. Metz, Deale

    Your cactus is probably growing too fast. How often are you watering it? The more you water it, the faster it will grow. Cactus plants should be irrigated only once every three or four weeks, depending on the species. We generally repot cactus plants only every two to three years. My wife has a cactus that has not been repotted for at least five years. It is the kind of plant that prefers neglect.
    The brown tissue is mature tissue just like bark on a tree. As the tree ages, the oldest bark near the base of the trunk develops a different color and the cells thicken.

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    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.