Pruning is one of the most misunderstood aspects of gardening. Those of us who are more gentle and sensitive may not want to cut into a living tree or shrub at all, leaving the pruning to nature. Others who identify with aggressive measures see control as the goal and prune as if they were beating back the jungle. However all externally applied concepts should be relinquished in preference to an approach which begins with the needs and repair systems of the plants themselves. What’s wrong with the let-it-be approach? Nothing if you don’t mind the tree/shrub having half as long to live and finding broken branches hanging and fallen after big storms. Dead branches in particular should not be left too long because they are entries into the tree for decay and diseases; the bark, like our skin, is a protective organ. And what is wrong with hacking back a shrub or tree that is overgrown? Nothing if you don’t mind the tree/shrub living half as long because the stress caused by this approach weakens the plant and makes it vulnerable to decay, diseases and insect damage; nothing is wrong if you don’t mind the forest of sucker growth that follows overpruning; no problem, if you don’t mind your wife being mad at you for years every time she looks at its tortured form.
The preferable alternative to these conceptual approaches begins with the “organic” needs and systems of the tree or shrub, especially their self-repair systems. According to world-renowned tree expert Alex Shigo who has spent a lifetime studying trees, plants don’t heal their wounds; they wall them off so decay can’t spread , and they close over the opening. In the past arborists made flush cuts, creating elliptical cuts flush with the source branch. Alex Shigo’s research has revealed an area he calls the branch collar which, if removed with a flush cut, also removes a protection zone allowing decay to spread much further. However if a pruning cut starts in the branch crotch and slopes outward at an angle, leaving a more circular cut, the protection zone is left intact and decay is contained.(see diagrams) This approach has been known for about 15 years and has been mandated by many cities. Still, the branch collar can look very different in different varieties and circumstances or can be without any swelling to distinguish it. Even very knowledgeable people who are training others are still recommending leaving a short stump; this is wrong. Even a ½” long stub has no way of feeding itself, becomes dead wood and conducts moisture and decay into the main branch, taking years and years to close over, if it ever does.
Besides making proper cuts, respecting a tree or shrub’s organic needs includes helping to develop a strong structure, removing dead and broken branches and thinning for strength and light penetration. If these needs are so organic, why don’t trees and shrubs do these things themselves? Trees do slough off dead and broken branches, but it often takes years to happen and meanwhile decay often has gone a long way. And why do trees need to have their structures improved? Because the majority of trees growing here did not evolve here and because they are often over watered and over-fertilized so that their long thin branches commonly break in Colorado winds and wet snows. If you spend years walking out on limbs as I have done, you get a feel for what proportion of diameter to length is structurally strong for each kind of tree. However for the practical purposes of the average gardener, it is enough to know that roundish and conical forms are the strongest , and that what is beautiful and balanced looking is usually what is stable structurally. In general, removing even 10%-20% of the length of a leggy branch will reduce the leverage significantly. And why do we need to do thinning? Trees and shrubs do thin themselves; when sunlight can no longer penetrate into the interior of a tree, small branches will die and if it is too dense a shade, only the outer portions of the tree will have leaves. So we can help the trees get light by thinning out rubbing and crowded branches, the majority of which can usually be under one inch diameter. Besides taking care of the trees’ needs, it may also be necessary to prune branches back from buildings and sidewalks, to lift up trees for clearance and to prune for gaining a view. If these functions cannot be accomplished without mutilating the tree/shrub, it is better just to remove it.
When is the right time to do pruning? This question has been debated for centuries and so far nobody’s opinion has been recognized as the “right” one. If a particular time were really bad, surely we would know about it by now. However here are a few good guidelines:
- The old adage “The right time to prune is when your tools are sharp” can be respected. A dull cutting tool not only takes a lot more effort to use, it is more dangerous and leaves a ragged cut which takes longer to close over.
- Prune flowering shrubs and trees soon after they bloom. It will do them no harm if pruned at other times, but they will not flower as well the following season.
- Prune grapes early in spring before they leaf out so they don’t “bleed” profusely and so the growth is in new wood which is productive for fruit.
- Old shrubs that are flattened, half-dead, mostly old wood that doesn’t flower or in general passed fixing up can be cut to the ground in early spring before the leaves come out. If the root system is still viable, they will shoot up rapidly and in 2 or3 years with some thinning of the sprouts, can make a new shrub and flower again. Shrubs for which this approach is effective are lilac, forsythia, mock orange, spirea, potentilla, Rose of Sharon, and butterfly bush (B. davidii).
- Roses are best pruned in early spring, some say “when forsythia blooms”. Here in Colorado that can be too early because pruning too early can stimulate growth that can then be frozen in later spring freezes. If the rose is new or lacks reserves, death of the new growth can be death of the plant. In general, floribundas and hybrid teas can be cut back to 10—12″,and shrub roses and old roses need only have dead wood removed, along with canes that are rubbing or in the way. Canes showing the swollen and scarred areas indicative of rose girdler should be cut off below the swelling and the canes burned or removed, not composted.
In the next issue I will discuss pruning of fruit trees and why I prefer to prune them in the dryer, warmer weather of the later seasons. I will also explore the controversial approach of tipping, and discuss shrub renewal pruning.