Pruning

Principles of Pruning

Over the years I've been asked hundreds of questions about pruning. It's a subject most people find overwhelming in scope, and daunting in practice, so they simply don't prune. Unfortunately, lack of pruning can sometimes be just as bad as poor pruning. Most books and articles focus on the how to and timing aspects of pruning but ignore information on tree health and growth, two basic principles that should be the cornerstone of pruning.

All organisms are living systems that thrive as long as they can adequately replenish energy, maintain health, and defend themselves against disease and injury. Human and plant systems do this in fundamentally different ways. Injured or dead human cells can be replaced with healthy cells; wounds often heal with little remaining visible or internal damage. Much of the human system works by regenerating new cells in place of old.

Trees don't heal from injury as we define healing. They compartmentalize injury and generate new growth elsewhere. Special cells create barriers that seal off and protect undamaged cells from a nearby injury. On the exterior of a tree, we can see wound-wood surrounding a good pruning cut. But to adequately compartmentalize the wound, a similar barrier is formed inside the tree around the entire interior branch core. This interior compartmentalization prevents the remains of a pruned, dead, damaged or diseased branch from becoming a conduit for pathogens and rot to enter into the tree. When this system works, it can protect against natural or human-inflicted injury.

Coping with injury requires use of energy, as do the basic functions that maintain growth and life. Photosynthesis is how a tree produces energy, but for pruning purposes, it's more important to know where energy is produced. Trees have two types of mass--static and dynamic. Dynamic mass is the energy producing, growing parts: new branches, leaves, buds, and vigorous new roots and young bark. Static mass is the non-energy producing structure: thick, woody older bark, woody roots and interior wood. The ratio between the amount of static and dynamic mass changes over the life of a tree. Younger trees have a high ratio of dynamic to static mass, older trees the opposite. As a tree gets larger and older, it begins to increase its mass beyond the energy producing capabilities that maintain optimum health. The vast amount of energy needed to sustain life in older trees makes them more susceptible to the additional stress of injury or disease. However, greatly reducing their mass and encouraging new growth will not make a big, old tree into a small, healthy one. The aged, static system can't adapt to such radical treatment. But a tree can be kept small and healthy for a long time if properly pruned on a regular basis.

The smaller, high energy producing systems of young trees can more easily spare energy to injury--pruning. Which is why pruning for shape is encouraged when trees are young. In fact, pruning should be a regular part of annual maintenance for any tree. A good clean pruning cut is always preferable to a broken, ripped or diseased branch. Removing poorly angled branches when they're small also reduces the chance of greater damage later on, when the tree can less afford the injury.

A dissected tree tells the story of every lost branch and wound throughout its lifetime, just like growth rings tell its age. Ideally, interior scars are walled off by the tree's defense system, which also protects and sustains its system for liquid transport and structural support. If this defense system fails, interior problems can worsen, and the tree can die, sometimes over decades, from the inside out. Potential problems can be spotted if growing fungus or oozing liquids emerge from old wounds. While it's a good idea to remove rot from old wounds, don't damage wound-wood in the process and don't use sealants. Sealants often protect and encourage rot more than they help the tree.

Pruning Trees

Much has been written about the angle of pruning cuts. You can forget most of it if you understand about branch collars. Branch collars are the clusters of living cells that surround the base of branches. All woody plants have branch collars, and all are slightly different in shape and size, even on the same tree. Whether the branch is large and attached directly to the trunk of a tree, or just a small branch on a larger branch, it will have a branch collar. Some are more obvious than others, but most resemble a slight bulge, or ridge, that circles the entire base of the branch. When a branch is removed, the tissue in the branch collar will provide a protective barrier that seals the wound off from the surrounding bark. If that tissue is damaged and unable to surround the wound entirely, the tree will be less able to compartmentalize the wound.

A proper pruning cut is made as close to the branch collar as possible without damaging it. The angle is determined by the shape of the branch collar, not by some predetermined formula. The cells that form the protective wound-wood barrier are only present in the branch collar; they don't form several inches out the branch. If the cut is made too far from the branch collar, the wound is left less protected. Dead or dying branches should be pruned with the same technique.

Now that you're comfortable with the idea of branch collars, you have to go one step farther. Some branches grow at such an acute angle that they grow up into the tree before leaving it completely. Included bark or a branch bark ridge develops between the branch and the tree trunk. Start your pruning cut as close as possible to the branch bark ridge, on the branch side, without damaging it. The non-acute angle part of the branch usually exhibits the regular, bulging branch collar attributes to guide the rest of your cut.

Another exception is forked or codominant stems: two stems growing at the same rate from the same position. Don't buy a young tree with codominant stems. If you have an existing tree with a codominant main trunk, it may need to be professionally braced. On a younger tree or branch, the less dominant branch can be thinned to encourage the dominant branch to become stronger. Once that happens, the bark ridge becomes stronger and more defined, and the lesser stem can be pruned out.

Most deciduous trees are best pruned near the end of their dormant season. If this is not possible, prune immediately after the leaves have matured, in the summer. If possible don't prune when leaves are forming or falling. If you are timid about pruning and have a weed tree, practice pruning on it. Whether you prune or not, if you own trees you should become familiar with the terms and conditions I've talked about. Look for the different shapes and sizes of branch collars. Try to find a codominant branch or trunk and look for the ridge bark. If you find problems, hire a professional.

Pruning Shrubs

Shrubs, for the most part, can be thought of as indefinitely young trees with a high ratio of dynamic growth. Most shrubs are multi-stemmed and do not rely on one main trunk for their life support system and structure, and can often be pruned back drastically with good result. When pruning back entire branches follow the same technique as in trees.

Shrubs are often pruned mid-branch, whether for deadheading flower clusters or to encourage branching. When a branch is pruned back to a dormant bud, growth is stimulated and a new branch sprouts in the direction the bud was pointing. This is the place where prescribed angle theory becomes relevant. When pruning back to a dormant bud, the cut should slope away from the bud and be made about a 1/4" above it. This sheds water away from the bud and gives it ample room to swell and develop. It is extremely rare that a wrong angled cut, or one that is more than 1/4" from the bud, will irrevocably damage a shrub. However, with a little concentration and practice the right cut becomes a good habit that takes no more time to establish than a bad habit. Needless to say if the bud is damaged by too close a cut, or a rip, it won't grow. One very important note on pruning a lopsided shrub: prune the weaker side. Mid-branch or tip pruning encourages growth. If you want to reduce size, prune or thin out entire branches.

Shrubs that bloom in the spring on old wood should be pruned soon after the flowers fade. New growth needs to mature over the summer and fall in order to produce flower buds for the next spring. If you prune too late in the summer the resulting new growth won't have time to produce mature flower buds. Winter pruning doesn't harm the shrub but will reduce bloom the following spring because you will be cutting off flower buds along with the branches. Shrubs that bloom on the current season's new growth should be pruned in early spring. This encourages more flowers and can correct size and shape. Weak growth and damaged branches should be removed as well.

Your pruning routine should always include taking out overcrowded, dead or damaged branches. Branches that rub against each other or cross at odd angles should be removed or untangled. Air and light circulation into the interior of a shrub maintains good health and bloom. This usually requires thinning out old, large branches. Tag the ones you want to keep and carefully follow to their origin the ones you want to remove . Opening up a dense, overgrown shrub can often make a large blob into a handsome structure. Remember that branches cut back to dormant buds will encourage new growth and will grow in the direction the bud points. If you have a shrub destined to be removed, practice pruning on it. Not only will you gain confidence with pruning, but you might give it a new look that makes it worth keeping.