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 Pruning Mature Trees
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When pruning mature trees, do it right or not at all
Round Rock Leader 
Saturday, February 21, 2004
By Emsud Horozovic, City of Round Rock Forestry Manager

Tree pruning and common sense go hand in hand. Although very often in our community, when we look at the way that trees are pruned, we wonder where the common sense went. Perhaps it went to the brush pile. By following some basic rules on pruning trees, we can have a profound effect on the shaping and maintenance of our trees.

Pruning is the most common, yet often unnecessary, tree maintenance procedure. Trees are pruned for three main reasons: safety, health, and aesthetics. Improper pruning can cause damage that will last for the life of the tree, or worse, shorten the tree's life. It is better to not prune at all rather than prune incorrectly.

Reasons for Pruning

Pruning for safety involves pruning branches that could fall and cause injury or property damage, or trimming branches that could interfere with lines of sight on streets or driveways or utility lines.  Safety pruning can largely be avoided by carefully choosing species that will not grow beyond the space available to them and are suitable to the sight.

Pruning for health involves removing diseased or insect-infested wood, thinning the crown to increase airflow and reduce pest problems, or removing crossing or rubbing branches.  Also removing broken, damaged, and dead limbs will encourage wound closure. 

Pruning for aesthetics involves enhancing the natural form and character of trees or stimulating flower production.

Before you try to impress the world and the trees with your chainsaw skills, keep the following in mind:

  1. Each cut has the potential to change the growth of the tree.  Always have a reason/purpose in mind before a cut is made. 
  2. Proper technique is essential.  Poor pruning can cause damage that lasts for the life of the tree.  Learn where and how to make the cuts before picking up the pruning saw.
  3. Trees do no heal the way people do.  When a tree is wounded it must grow over and "compartmentalize" the wound.  In effect, the wound is contained within the tree forever.
  4. As a rule, small cuts do less damage to the tree than large cuts.  This is why proper pruning (training) of young trees is critical, and I’ll address that in next month’s article.

Common reasons for pruning are to remove dead branches, to remove crowded, crossing or rubbing limbs, and to eliminate hazards. Trees may also be pruned to increase light and air penetration to the inside of the tree's crown or to the landscape below. In most cases, mature trees are pruned as a corrective or preventative measure.

Routine thinning does not necessarily improve the health of a tree. Trees produce a dense crown of leaves to manufacture the sugar used as energy for growth and development. Removal of foliage through pruning can reduce growth and stored energy reserves. Heavy pruning can be a significant health stress for the tree.

When to Prune

Most routine pruning to remove weak, diseased, or dead limbs can be accomplished at any time during the year with little effect on the tree. As a rule, growth is maximized and wound closure is fastest if pruning takes place between November and February, the dormant period for Central Texas trees.

Due to oak wilt disease, avoid pruning of all oak species between February 1 and June 1. Heavy pruning just after the spring growth flush should be avoided. This is when trees have just expended a great deal of energy to produce foliage and early shoot growth. Removal of a large percentage of foliage at this time can stress the tree.

You may need to remove branches with narrow crotch angles. Wide crotch angles are generally 40 degrees to 90 degrees and are very strong as thee branch is composed of solid wood. Crotch angles less than 40 degrees are weak and should be pruned when the tree is still young. In older, established trees, bracing and cabling may be the only option.

Making Proper Pruning Cuts

It is difficult to describe the proper pruning techniques for each tree species. Before you start, avoid the following:

  1. Say no to tree topping, a common butchering practice designed to encourage growth. Tree topping damages the strength and structure of a tree.
  2. Do not make stub cuts, leaving a portion of a branch sticking out. This allows rot to enter into the tree and the tree cannot close over the stub.
  3. Do not limb up, which is removal off all of the lower limbs. Lower branches should be left and are needed to encourage the growth of the trunk. They can be removed gradually and only if they are a hazard.
  4. Do not make flush cuts, which is cutting into the trunk or a major branch while pruning. This damages the tree’s protective zone, making it more difficult for the wound to close.
  5. Don’t use pruning paint or so called wound dressings, except for tree pruning cuts on all oak species at all times due to the threat of oak wilt disease.

Pruning cuts should be made just outside the branch collar. The branch collar contains trunk or parent branch tissue and should not be damaged or removed. If trunk collar has grown out on a dead limb to be removed, make the cut just beyond the collar. Do not cut the collar.

If a large limb is to be removed, its weight should first be reduced. This is done by making an undercut about 12-18 inches from the limb's point of attachment. A second cut is made from the top, directly above or a few inches further out on the limb. This removes the limb leaving the 12-18 inch stub. The stub is removed by cutting back to the branch collar. This technique reduces the possibility of tearing the bark.

Pruning Techniques

Specific types of pruning may be necessary to maintain a mature tree in a healthy, safe, and attractive condition.

Cleaning:
the removal of dead, dying, diseased, crowded, weakly attached, and low-vigor branches from the crown of a tree.
Thinning:
the selective removal of branches to increase light penetration and air movement through the crown. Thinning opens the foliage of a tree, reduces weight on heavy limbs, and helps retain the tree's natural shape.
Raising:
removes the lower branches from a tree in order to provide clearance for buildings, vehicles, pedestrians, and vistas.
Reduction:
reduces the size of a tree, often for clearance for utility lines. Reducing the height or spread of a tree is best accomplished by pruning back the leaders and branch terminals to lateral branches that are large enough to assume the terminal roles (at least 1/3 the diameter of the cut stem). Compared to topping, this helps maintain the form and structural integrity of the tree.

How Much Should Be Pruned?

The amount of live tissue that should be removed depends on the tree size, species, and age, as well as the pruning objectives. A common mistake is to remove too much inner foliage and small branches. It is important to maintain an even distribution of foliage along large limbs and in the lower portion of the crown. Over-thinning reduces the tree's sugar production capacity and can create tip-heavy limbs that are prone to failure.

Mature trees should require little routine pruning. A widely accepted rule of thumb is never to remove more than 1/4 of a tree's leaf bearing crown. In a mature tree, pruning even that much could have negative effects. Removing even a single, large-diameter limb can create a wound that the tree may not be able to close. The older and larger a tree becomes, the less energy it has in reserve to close wounds and defend against decay or insect attack. The pruning of large mature trees is usually limited to the removal of dead or potentially hazardous limbs.

Hire an Arborist

Pruning large trees can be dangerous. If pruning involves working above the ground or using power equipment, it is best to hire a professional arborist. An arborist can determine what type of pruning is necessary to improve the health, appearance, and safety of your trees. A professional arborist can provide the services of a trained crew, with all of the required safety equipment and liability insurance.

There are a variety of things to consider when selecting an arborist:

  • Membership in professional organizations such as the International Society of Arboriculture (ISA), the Tree Care Industry Association (TCIA), or the American Society of Consulting Arborists (ASCA).
  • Certification through the ISA Certified Arborist program.
  • Proof of insurance.
  • A list of references (Don't hesitate to check.)
  • Avoid using the services of any tree company that:advertises topping as a service provided. Knowledgeable arborists know that topping is harmful to trees and is not an accepted practice.

  

Forestry Manager Emsud Horozovic
Phone: 512-218-5540





 


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