Pruning young trees

by William Scott
Wednesday, May 28, 2008 9:08 AM MDT

City Forester

Proper pruning is essential in developing a tree with a strong structure and desirable form.

Trees that receive the appropriate pruning when they are young will require much less pruning when they mature. Pruning young trees is easier, faster and safer.

Keep these few simple principles in mind before pruning a tree:

* Each cut has the potential to change the growth of the tree. Always have a purpose in mind before making a cut.

* Proper technique is essential. Poor pruning can cause damage that lasts for the life of the tree. Learn where and how to make proper cuts.

* Trees cannot heal the way people do. When a tree is wounded, it must grow over and around the wound. As a result, the wound is contained within the tree forever.

* Small cuts do less damage to the tree than large cuts. For that reason, proper pruning of young trees is critical. Waiting to prune a tree until it is mature can create the need for large cuts that the tree cannot easily close.

Making the cut

Where you make a pruning cut is critical to a tree’s response to that wound. Make pruning cuts just outside the branch collar.

Because the branch collar contains trunk and branch tissues, the tree will be damaged unnecessarily if you cut the branch off flush with the trunk remove or damage the collar. In fact, if the cut is large, the tree can suffer internal decay.

First, undercut to prevent tearing the bark. Second, cut to remove the weight of the branch. Then cut just outside the branch collar to finish a proper pruning cut.

If a permanent branch is to be shortened, cut it back to a lateral branch or bud that is at least one-third the size of the branch you are shortening. Cuts made between buds or branches may lead to stem decay, sprout production and misdirected growth.

Pruning tools

When pruning trees, it is important to have the right tool for the job. For small trees, most of the cuts can be made with hand pruning shears.

The scissor-type, or bypass blade hand pruners, damage the tree less during a cut, and are recommended over the anvil type that can crush the branch. Cuts larger than one-half inch in diameter should be made with bypass type loppers or a pruning saw.

Chainsaws should be considered only as a “last resort” tool. Never use hedge shears to prune a tree.

Whatever tool you use, make sure it is kept clean and sharp. Tools also should be sanitized between uses to avoid the possible spread of disease. A 12 percent bleach solution, alcohol or Lysol spray all are acceptable disinfectants.

Establishing a strong structure

A good structure of primary scaffold branches should be established while the tree is young. The scaffold branches provide the framework of the mature tree.

Properly trained young trees will develop a strong structure that requires less corrective pruning as they mature.

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.

Naturally, those factors vary with the growth habit of the tree. Good pruning techniques remove structurally weak branches while maintaining the natural form of the tree.

Trunk development

For most young trees, maintain a single dominant leader growing upward. Do not prune back the tip of this leader. Do not allow secondary branches to outgrow the leader.

Sometimes a tree will develop double leaders, known as co-dominant stems. Co-dominant stems can lead to structural weaknesses, so it is best to remove one of the stems while the tree is young.

The lateral branches growing on the sides contribute to the development of a sturdy well-tapered trunk. It is important to leave some of these lateral branches in place, even though they may be pruned out later.

These branches, known as temporary branches, also help protect the trunk from sun and mechanical injury. Temporary branches should be kept short enough not to be an obstruction or compete with selected permanent branches.

Permanent branch selection

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 14 feet of clearance for traffic. Most landscape trees require only about eight feet of clearance.

The height of the lowest permanent 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 wind break may be allowed to branch low to the ground.

Most large-growing trees in the landscape eventually must be pruned to allow head clearance.

The spacing of branches in the tree is very important. Branches selected as permanent scaffold branches must be well spaced along the trunk. Maintain radial balance with branches growing outward in each direction.

A good rule of thumb for the vertical spacing of permanent branches is to maintain a distance equal to 3 percent 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.

Some trees have a tendency to develop branches with narrow angles of attachment and tight crotches. As the tree grows, bark can become enclosed deep within the crotch between the branch and the trunk. Such growth is called included bark.

Included bark weakens the attachment of the branch to the trunk and can lead to branch failure when the tree matures. You should prune branches with weak attachments while they are young.

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 two-thirds (more if possible) of the foliage on the tree.

Newly-planted trees

Pruning of newly-planted trees should be limited to corrective pruning. Remove torn or broken branches, and save other pruning measures for the second or third year.

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.

Wound dressings

Wound dressings, such as paint or tar, once were thought to accelerate wound closure, protect against insects and diseases and reduce decay.

However, research has shown that dressings do not reduce decay or speed closure and rarely prevent insect or disease infestations, and can, in fact, increase the amount of time the tree takes to close the wound.

For questions about your trees, feel free to contact the City of Casper, Urban Forestry Section, at 235-8309 or e-mail your city forester at wscott@cityofcasperwy.com.

Print this story   |   Email this story