Landscape trees need proper care and management throughout their lives, and one of the most important tree management practices is pruning. When done properly, pruning can improve the health and structure of trees, and provide a safer environment for people, pets, and property. 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 main reason to prune young trees is to develop good branch structure and tree strength. Removing weak branches and correcting poor form when branches are small, will minimize the size of the pruning wounds. Early pruning will also promote strength and balance that will make a tree less susceptible to damage from wind, ice, and snow storms. Attention to developing good structure is most critical in the first 15-20 years of a tree's life.
When To Prune
The best time to prune is in mid to late winter (January-March). When pruned during this time of the year, the tree will begin responding to the wounding early in the spring. A coniferous tree planted in a suitable site, will need minimal pruning throughout its life.
Pruning at other times of the year will not hurt a tree; however the process of sealing the wound may be slowed. Do not prune during the spring from bud break through leaf expansion, and during the period of leaf color change in the fall. A tree is going through major changes at these times, and branch removal can reduce the vigor of a tree. One species where timing of pruning is critical, is oak. The pathogen that causes the disease Oak Wilt can be transmitted to open wounds by a small beetle. Avoid wounding (pruning) oak between early March until late July.
Training Young Trees
Limit pruning of newly planted trees to the removal of dead and broken branches or the correction of multiple leaders. Begin developmental pruning of deciduous trees 2-3 years after planting. Other key things to remember when pruning young trees are to:
- Know the general growth habit of a tree before beginning.
- Leave the temporary lower branches on the tree until they reach 1 inch in diameter to increase trunk growth and root development.
- Always leave 70 percent of the tree height with live branches (see Figure 1).
- Avoid removing lower branches too quickly, keeping lower branches longer allows for larger and stronger tree trunks (see Figure 2).
- Concentrate efforts on removing crossing, rubbing, broken, diseased and weak-angled branches in the upper portion of the tree.
- Eliminate double leaders and basal sprouts.
- Develop one main leader on shade tree species such as: oak, maple, ash, and linden.
- Concentrate efforts on removing rubbing and competing branches on species such as crabapple.
- Space permanent branches 15-35 inches apart.
- Remember developmental pruning is an on-going process over the first 15-20 years of a tree's life.
How to Make a Pruning Cut
Before deciding which branches to remove, always examine the tree carefully. Before making a pruning cut, identify the branch bark ridge and branch collar. The branch bark ridge is where the branch and trunk tissue meet. The branch collar is the swollen area just outside the branch bark ridge (see Figure 3).
- Do not cut behind the collar and branch bark ridge creating a "flush cut". Removal of these two structures impedes the tree's ability to respond to the wound, which increases the chances of decay development.
- Do not leave a stub.
- Do not top a tree, which is the indiscriminate removal of branches without regard to the location of lateral branches or buds (see Figure 6).
- Always remove branches back to their point of origin or to a side branch of sufficient size to assume dominance.
The best indicator of the proficiency of your 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 existing wound (see Figure 7).
Note: If callus tissue develops only on the sides of the wound, this may indicate the pruning cut was too close to branch bark ridge and branch collar.
Proper pruning is the key to good wound closure. A number of studies have shown that the use of wound dressings or paints does not speed up the tree's ability to seal a wound. In most cases, pruning paints create a more favorable environment for decay causing organisms compared to doing nothing at all. At this time pruning paints are no longer recommended.
For additional information check out this tree pruning publication from Iowa State University Extension