Raise 'em RIGHT
As I write this article, the Kansas City metropolitan area has just experienced an ice storm the likes of which the area has never seen. Freezing rain fell over a two-day period, accumulating as much as 2 inches of ice. About 350,000 utility customers lost power. It would take more than a week to fully restore electricity. Cleanup costs approached $30 million in Kansas City, Mo. alone. That amount does not include all the surrounding communities that make up the metropolitan area. The total could easily exceed $60 million.
But that's only the tip of the “iceberg.” While the loss of power can be a serious inconvenience and hardship, the more devastating long-term impact of this storm is the damage and total loss of thousands of trees throughout the area. Once all the dust has settled, another imposing task is before us. What should be done with the high-risk trees that remain, and how do we begin replacing all the trees that have been lost?
Certainly, many of the failures that occurred were unavoidable. However, a great many could have been prevented. The major contributing factors that lead to failures under the tremendous ice loads were:
That is soft-wooded species (for example, silver maple) failed much more than strong-wooded species (for example, bur oak).
Wounds, interior decay and other pre-existing weak points.
Tree and branch structure
Co-dominant and multi-trunk trees, weak (narrow) branch attachments, and V-shaped crotches and forks contributed to failures.
Failures due to the last factor listed above can be greatly reduced by timely, judicious pruning throughout a tree's life. Many articles in periodicals and manuals refer to “training and formative” type pruning of young trees. While this is very important — and it's what I'll focus on in this article — bear in mind that it's not the only time in a tree's life when you should consider these principles.
Back to basics
Let's first consider some basic facts about trees. Most tree species evolved in forest communities. Close spacing results in single stem (trunk) trees with little lateral limb development. Due to natural shading, most forest trees do not retain low limbs throughout their lives and rarely do multi-trunk or co-dominant stems occur in a naturally occurring forest. Take a walk in a “real” forest sometime and take note of the trees. What you will usually see are tall single-trunk trees with few or no low lateral limbs.
However, those same species planted in the open, sunny environment of our yards and along our streets will not generally develop the same way. Due to the sunlight that reaches the lower part of the canopy, the tree will retain these lower limbs. This profusion of branches creates many potential points of structural failure. You must intervene with proper pruning if you are to prevent future limb and trunk failures from occurring.
Does this mean that landscape trees should look like telephone poles? Am I suggesting that all lateral limbs be removed? Absolutely not. But realizing that most trees have evolved to be single-stem individuals growing in forests is a good beginning to understanding why they often don't fair so well when we grow them in our urban environments.
Know your trees
Trees grow in a variety of shapes (form) and sizes. To know how to prune a particular species, you must have a good understanding of its “normal” growth habit and branching tendency. For example, pin oak and sweetgum have strong apical dominance, with usually just one central leader or trunk, and wide branch angles with good strength. Other than removing the occasional co-dominant stem, pruning is relatively easy.
However, consider redbuds and dogwoods. These trees growing in a “real” forest are usually single-trunk individuals due to living in the understory of dominant oak and hickory forests. But line them out as whips in a nursery — which is how they're typically grown — and what do you usually get? Multi-branched, poorly structured individuals that usually fail long before they reach maturity after being planted in our yards and parks. Such species tend to require more careful and perhaps aggressive pruning early in life to develop a stronger structure.
Listed in the table below are some species commonly planted in the Midwest and East that require close monitoring for structural pruning needs. These species and their cultivars have tendencies toward co-dominant and multi-trunk development, forks and V-shaped crotches or weak (narrow) branch attachments. This list is, of course, far from complete, but these widely planted species account for a large portion of the landscape trees that tend to develop poor branching structure when grown in urban environments.
The first 10 years … and more
The first 10 years of a tree's life in its new home is a critical period that can determine whether it lives to maturity or dies prematurely due to some type of structural failure. Unfortunately, when formative pruning is neglected early in a tree's life, it often loses its central leader or develops some other severe structural problem with its main scaffold limbs. In many such cases, it is effectively too late to correct the problem without making unduly severe cuts. The five steps I present below go a long way toward ensuring a good start for young trees.
Step 1: Choosing the tree
Select a good specimen tree from the nursery. This can be a challenge in itself. Look for a single stem/trunk with few low lateral branches. Avoid trees that fork within 5 feet of the ground. For species such as redbud and dogwood, this can be difficult. But it is not impossible. Be choosy!
Step 2: 1 - 2 years
Within the first two years of planting, keep pruning to minimum. Leaves are the “factories” of the tree that manufacture food for growth and recovery. This is especially important at this time in a tree's life, when rapid root growth ensures rapid establishment and recovery from transplanting. However, you should always remove dead, broken or damaged limbs, and also prune as needed to maintain and encourage a central leader.
Step 3: 2 - 5 years
From two to five years after planting, pruning should begin in earnest. Continue to encourage the central leader, removing any co-dominant stems and any lateral limbs which may be growing upright rather than outward. Retaining some temporary lower limbs is beneficial in protecting the lower trunk and assists in developing trunk taper and strength. A good rule of thumb is that lateral limbs on young trees should always be smaller than the trunk by at least half. Those that approach the size of the main trunk may threaten its dominance and should be removed sooner rather than later.
Often, people prune trees at this stage with a mind toward making them look good now, rather than later. Thus, for example, they may be reluctant to remove unfavorable limbs because they don't want to lose fullness. This is a mistake. It often results in retaining limbs that clearly are too low or create V-crotches because they provide nice shape or fullness now. However, retaining them may result in loss of the central leader. Or, they become too large to remove without serious injury to the tree.
It is better to be picky now about which branches you retain, even if the result is a young tree that doesn't look attractive for a couple of years. The branches you keep may be very small at this point, but they'll grow with no problem if you remove competing, less-desirable limbs. Remember, this is temporary and is a small price to pay for potentially adding decades to a tree's life, not to mention more attractiveness and less resources devoted to repairing damage to the mature tree later on.
Step 4: 5 - 10 years
The period five to ten years after planting is just as critical as the first few years. Unfortunately, it is at this point that most trees are left to fend for themselves. They're often too small to need the work of a tree service company, but too big for the average homeowner to work with. If the tree's shape looks good and the branches aren't poking you in the eye when you mow, then everything must be fine. At least, that seems to be a common attitude.
However, nothing could be further from the truth. Co-dominant stems can still develop higher up in the canopy of the tree. At this point, while the tree is still relatively young, these can easily be reached with pole pruners. Also, be diligent in removing low, temporary limbs before they get too large. Keep in mind the mature height and spread of the tree you are pruning. Removing lateral limbs 7 to 9 feet up may be adequate for a dogwood, but it is not at all high enough for something like a bur oak which may ultimately attain a height of well over 70 feet.
Step 5: Beyond 10 years
The particular species and the intended function of the tree in the landscape will determine how low you should allow permanent limbs to remain. For example, the lowest limbs of a pin oak along a residential street may need to be more than 20 feet from the ground, whereas a dogwood in a shade garden may only need to be 3 feet from the ground. Whatever the height of these permanent scaffold limbs, you should consistently use certain criteria to determine which branches to retain or remove.
Permanent limbs should always be smaller in diameter than the trunk at their point of attachment.
Select scaffold limbs that grow at a wide angle from the tree; in the range of 50 to 90 degrees is best.
Encourage adequate vertical spacing along the trunk between limbs. This can vary significantly between species. Spacing of 1 to 2 feet may be appropriate for some trees or as much as 6 to 10 feet on others. You must know the species you are dealing with.
By following the guidelines I've presented here, you'll help trees reach their full potential with good structural integrity. From time to time, weather extremes will occur. These place strong forces and strains on our trees, inevitably resulting in some failures. However, many of these failures can be avoided by using judicious, timely, corrective and formative type pruning throughout a tree's life, rather than crisis “cleanup and salvage” pruning once the damage has occurred.
Michael W. Dougherty is an urban forester and owner of Tree Management Co. (Lenexa, Kan.).
TREES WITH TENDENCIES FOR WEAK STRUCTURES
Ornamental pear (especially Bradford)
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