Plan the pruning of young shade trees for a healthy life
Round Rock Leader
Saturday, March 20, 2004
By Emsud Horozovic, City of Round Rock Forestry Manager
A new tree in your landscape holds the promise of cool shade on hot Texas afternoon, branches for hanging a swing, rustling leaves, and a bit of fall color. Like a youngster, your tree will need some discipline in order to live a long, productive life. You can think of careful pruning as a way of training your young tree to grow strong and healthy.
Good branch structure begins in the nursery. As mentioned in the December article, look for a tree with a strong dominant trunk and evenly spaced branches. Avoid double trunks and low, narrow crotches. If you select a well-formed and healthy tree, premature pruning should be minimized.
DROP THE CLIPPERS AND READ ON!
Pruning is more than just indiscriminately removing branches. Proper pruning includes knowing which branches to remove, when to do it, and how to minimize damage to the tree. The belief that trees should be pruned when planted to compensate for root loss is misguided. Trees need their leaves and shoot tips to provide food and the substances that stimulate new root production. Unpruned trees establish faster with a stronger root system than trees pruned at the time of planting.
Therefore, you should not do any corrective pruning the first year. Plan to start your pruning regimen before the second or third growing season. Give your tree a little pruning help every year during the first five or six years after it has been planted. After that, you can walk away from it for another three or five years. That said, there are some structural issues you need to understand before you can pick up those tools.
GETTING READY TO PRUNE: PLANNING
Before you begin to prune, look closely at the tree. Identify the dominant trunk, the temporary branches to be removed over time, the start of the first permanent limb, and any other branches that are rubbing or crowding. Picture how you want the tree to look when it is mature. Now, let’s look at the details.
The goal in training young trees is to establish a strong trunk with sturdy well-spaced branches. The strength of the branch structure depends on the relative sizes of the branches, the branch angles, and the spacing of the limbs. This will vary with the growth habit of the tree. Red oaks, for example, have a conical shape with a central leader. Elms and live oaks are often wide spreading without a central leader. Other trees, such as ash trees and Bradford pears, are densely branched. Good pruning techniques removes structurally weak branches while maintaining the natural form of the tree.
Nursery trees often have low branches that may make the tree appear well proportioned when young, but low branches are seldom appropriate for large growing trees in an urban environment. How a young tree is trained depends on its primary function in the landscape. For example, street trees must be pruned so that they allow at least 12-14 feet of clearance for traffic. Most landscape trees in yards, parks, and above sidewalks only require about eight feet of clearance.
The height of the lowest permanent branch (also called the lowest scaffold branch) is determined by the tree's intended function and location in the landscape. Trees that are used to screen an unsightly view or provide a windbreak may be allowed to branch low to the ground. Most large growing trees in the landscape must eventually be pruned to allow head clearance.
In selecting the lowest permanent branch, look for a stem with a diameter of one-half or less of the trunk diameter where the branch attaches to the trunk. You may want to label or lightly tie a piece of string to this branch so you can identify it later. If the tree is too small for you to select a branch at the desired height, then you’ll have to wait until the tree grows taller.
A good structure of primary scaffold branches should be established while the tree is young as these branches provide the framework of the mature tree. A good rule of thumb for the vertical spacing of permanent scaffold branches is to maintain a distance equal to 3% of the tree's eventual height. Thus, a tree that will be 50 feet tall should have permanent scaffold branches spaced about 18 inches apart along the trunk. Avoid allowing two scaffold branches to arise one above the other on the same side of the tree.
Additionally, the spacing of branches, both vertically and radially, in the tree is very important. Maintain radial balance with branches growing outward in each direction.
Once you have understood the tree’s potential, or know the general growth habit of a particular species of the tree, you are ready to start pruning, remembering the following:
• Each cut has the potential to change the growth of the tree. Always have a purpose in mind before a cut is made and remember how you wanted that tree to look.
• Small cuts do less damage to the tree than large cuts. This is why proper pruning (training) of young trees is critical. Waiting to prune a tree when it is mature can create the need for large cuts that the tree cannot easily close.
• When you identify the dominant leader, do not prune back the tip. Do not allow secondary branches to outgrow the leader.
WHAT SHOULD BE PRUNED
Torn, damaged, dead branches
Double Leaders: Maintain a dominant trunk for at least six-eight feet without a major fork. If the trunk divides into two or more relatively equal stems, favor one strong stem and remove the others. In some cases, you may need to do this gradually over several years. Cut one stem back to a lateral branch and let the other fill out and become dominant. After a year or two, remove the less dominant stem. The rule of thumb is to prune out no more than one third of the crown in a year, but on young trees you may be able to push the limit a little higher to remove a double leader.
Rubbing branches: Eliminate branches that are rubbing or will soon rub against another branch
Crowding: Give each branch room to grow with minimal competition for sunlight. Your goal is to have major lateral branches evenly spaced eight to ten inches apart along the trunk. However, avoid over-thinning the interior of the tree. The leaves of each branch must manufacture enough food to keep that branch alive and growing. In addition, each branch must contribute food to grow and feed the trunk and roots. Removal of too many leaves can starve the tree, reduce growth and make the tree unhealthy. A good rule of thumb is to maintain at least half the foliage on branches arising in the lower 2/3 of the tree.
Narrow Branch Angles: Sometimes two branches will form a narrow, v-shaped crotch. As the two stems grow thicker each year, bark may be trapped between the two stems, preventing them from forming a strong union. This is called “included bark.” Branches with included bark at the point of attachment are more susceptible to failure under load from wind or ice. Occasionally, otherwise healthy limbs on mature shade trees will fail on a clear, calm day due to included bark. If you see bark becoming pinched between a branch and the trunk, remove the branch before it grows large enough to become a hazard.
Sprouts and Suckers: Fast growing sprouts that shoot out of a trunk or main limbs have a weak point of attachment. If they are allowed to reach a large size, they may break during a storm, causing serious damage to the tree.
Temporary branches: While the tree is young, it may have small lateral branches along the main trunk. These branches contribute to the development of a sturdy well-tapered trunk. They also protect the trunk from sun and mechanical injury. You can leave them on the tree for the first few years as they help develop a thicker trunk. However, they should be short enough not to be an obstruction or compete with selected permanent branches, usually at a length of two to four buds for weaker branches, or one foot for more vigorous branches. Starting at the bottom, remove one or two of them each year until you reach the height of the first permanent limb.
Following these guidelines will ensure beauty, shade, a solid branch to swing on, and a home for birdies. The beauty of a nicely groomed tree will increase the value of your property as well.
For more information about the Forestry program and other helpful tree care and tree planting hints, visit our website through the Parks and Recreation Department at www.ci.round-rock.tx.us!
Forestry Manager Emsud Horozovic