Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
2/3/03
Contacts:
Paul Wray, Forestry Extension, (515) 294-1168, phw@iastate.edu
Jean McGuire, Continuing Education and Communication Services, (515) 294-7033,
jmcguire@iastate.edu
Yard and Garden Column for the Week Beginning Feb. 7, 2003
Pruning Trees: A Forester's View Point
Paul Wray
Extension Forester
Iowa State University
As a forester dealing with woodland owners, the subject of pruning often comes up, especially with respect to black walnut plantations. Pruning is one of those cultural activities in forestry that is expensive and time consuming. In forestry, pruning should be minimized as much as possible. Foresters often encourage landowners to prune no more than 100 trees per acre, and where possible, use "natural pruning" to produce valuable defect-free sawlogs.
In the urban forest, pruning is usually required to meet landowner goals and objectives. You prune for many reasons including clearance for people and traffic, to promote trees with better structure, for health and vigor of the tree, and for appearance.
Pruning the urban or community tree is not a one-time affair; pruning should start when the tree is very young and continue as needed throughout the life of the tree. Certainly, the first few years of pruning and training are most critical in the initial development and form of the tree in the landscape.
Pruning newly planted trees is often done incorrectly. First, plant good plant material and never prune the newly planted tree to compensate for lack of roots. Excessive pruning puts the newly planted tree at a disadvantage because the tree may not have sufficient leaf area to provide its requirements for growth, both above and below the soil surface. New plantings should only be pruned to correct damaged branches and major structural defects such as a multiple leader.
Pruning is very simple for large growing shade and conifer trees. For many trees, most of their branches formed during the first years of growth are temporary and will eventually be removed to meet the landowner's objectives for the tree. You only need to follow two simple rules.
Rule One: "Correct multiple leaders as soon as possible." The goal for large shade and conifers is to develop a tree in your yard which looks like a forester's sawlog tree. Maintain a single stem or leader as long as possible. Multiple leaders result in structural defects, such as included bark and weak branches. As the trees grow and become larger, they often break up in winds and storms.
Rule Two: For temporary branches or branches that will eventually be removed, "leave them on until they are one inch in diameter, and remove them before they are larger than two inches in diameter." By following this rule, you will maintain sufficient leaf area to provide the carbohydrates needed for top and root growth and the tree will be stronger. Removing the temporary branches before they are larger than two inches in diameter minimizes the size of the wound and speeds the healing process.
Many homeowners want immediate branch clearance, but this often results in excessive branch removal and the tree cannot provide the necessary carbohydrates for optimum growth. When evaluating the branches for removal, if the lower branches are less than an inch in diameter, leave them on the tree; if a branch up in the tree is approaching two inches in diameter, prune it. Pruning can be done within the crown; branch lifting does not need to be uniform. Follow Rule Two.
Pruning too much too early is one of the most common pruning mistakes. Leaving sufficient branches and leaves will provide the carbohydrates needed for tree growth and result in strong trees. To evaluate the strength of the stem of a young tree, look at the taper of the trunk. A tree with good taper is one that has a much larger diameter at the base of the trunk than at the top of the tree. Trees without taper are weak because carbohydrates are depleted before they reach the lower trunk and, more importantly, the root system. Trees with sufficient leaf area produce good trunk and root growth, resulting in a trunk with a very strong taper.
Following these rules will result in a tree that will last longer and
perform better in the landscape.
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Editors:
Four color photos, suitable for publication, are available at right.
Caption: Branch before (308K) |
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| Caption: Branch after (188K) |  |
| Caption: Tree before (512K) |  |
| Caption: Tree after (264K) |  |
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