Extension Communications
3614 Administrative Services Building
Ames, Iowa 50011-3614
(515) 294-9915
2/7/00
Contacts:
Mark Vitosh, Forestry Extension, (515) 294-6739, mvitosh@iastate.edu
Elaine Edwards, Extension Communication Systems, (515)
294-5168, eedwards@iastate.edu
Yard and Garden Column for the Week Beginning Feb. 11
Good Time For Pruning Young Shade Trees--Especially Oaks
By Mark Vitosh
Extension forester
Iowa State University Extension
The main reason to prune young shade trees is to develop good branch structure and strength to make the tree less susceptible to wind, ice and snow storm damage. Other important reasons for pruning young trees include the removal of dead, diseased and dying branches to improve tree health, and to create clearance between lower branches and the tree's surroundings.
A good time to prune deciduous (shade) trees is mid-winter (January-March) before the buds begin to open in the spring. During this time of the year, insect and disease-causing pathogens are at low levels, and without any leaves it is easier to see the structure of the tree. The timing for pruning oak is critical.
Avoid wounding (pruning) oaks from March 15 until the end of July to help prevent the spread of the disease oak wilt. Oak wilt is caused by a fungus, which invades the water-conducting vessels of the tree and usually results in death. Trees in the red oak group ( i.e., red, pin and black) are the most susceptible to this fungus, but trees in the white oak group (i.e., bur, white and swamp white) also are susceptible. One way the fungus is spread from an infected oak to a healthy oak of the same species is underground, through roots that grow together or graft, allowing the flow of material between trees. This fungus also can be transported above ground by sap-feeding beetles attracted to open wounds on healthy oaks. The best way to avoid above-ground spread is to avoid any type of wounding during the times mentioned above. Since now is a good time to prune oaks and other deciduous trees, let's talk about how to do it.
Pruning immediately after a new tree has been planted should be limited to the removal of dead or broken branches and double leaders (competing main stems) to maintain one single main stem. Actual developmental pruning of deciduous trees should begin 2-4 years after the tree has been planted or when lateral (side) branches are between 1 to 2 inches in diameter. Pruning should continue until the trees are mature.
Training a strong well-developed tree does not happen overnight. Below are a few factors to consider as you begin to prune your younger shade trees.
* Concentrate pruning efforts on removing crossing, rubbing, broken, diseased and narrow-angled branches, and double leaders in the upper portion of the tree.
* Always leave 70 to 75 percent of the tree height with live branches. Avoid removing lower branches too quickly.
* Leave the temporary lower branches on the tree until they reach 1 inch in diameter to increase trunk growth and root development, and then remove them before they reach 2 inches in diameter to minimize the size of wound.
* Before removing a branch, identify the branch bark ridge and branch collar. The branch bark ridge is where the branch and trunk tissue meet. This area is usually indicated by a ridge of bark between a branch and the trunk. The branch collar is the swollen area just outside the branch bark ridge.
* To avoid bark tearing, remove the weight of the branch before making the final cut just outside the branch collar.
* Avoid "flush cuts" or cuts behind the collar and branch bark ridge. Removal of these two structures interferes with the tree's ability to respond to the wound, increasing the chances of decay development in the tree.
* Avoid leaving a stub.
* Do not top a tree, which is the indiscriminate removal of branches without regard to the location of lateral branches or buds.
* Pruning paints or wound dressings are no longer recommended.
* The best indicator of proper pruning is the development of wound closure tissue on the tree. Usually within a year after branch removal, a ring (donut-shaped) of callus tissue will begin to develop around the wound of a properly pruned branch.
Before you head outside to start pruning, remember, developing strong and healthy young trees is accomplished over a number of years and not all in just one afternoon. More information on proper pruning of young trees can be found in Iowa State University Extension pamphlet SUL-5, Pruning Trees and Shrubs, which is available at your county extension office.
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