How to Prune Properly

When we plant a tree, we hope that it will grow tall and straight; that it will have a full, healthy crown with strong, well-spaced branches; that it will cast a broad expanse of sheltering shade; that it will resist damage by wind and ice; and that it will be easy to maintain. Without proper pruning, however, a tree can become unhealthy and expensive to maintain.

An unmaintained or poorly maintained tree is more likely to become hazardous, with branches that break during storms, have weak and unsightly shoots, and interfere with traffic, pedestrians and overhead wires. To ensure that Portland’s urban forest continues to enhance our daily lives, we must properly care for the young trees we plant.

Before pruning any street tree or other publicly owned tree, you must obtain a Street Tree Pruning Permit from Urban Forestry. 

For Private Trees a permit is only required for Native trees in c, p, or v overlay zones.

Street Tree Pruning Standards and Brochure

When Do I Prune My Tree?

You can prune a tree anytime of the year. Some say it’s best to prune a tree “any time the tools are sharp.” However, understanding how your tree will respond to pruning will help you make the best decisions for your tree.

Generally speaking, the following guidelines apply:

• Broken and dead branches as well as root suckers can and should be pruned at any time of year
• Because most trees in Portland experience seasonal dormancy, pruning live branches during the winter months encourages vigorous new growth in the spring without depriving the tree of actively photosynthesizing tissues
• Limited pruning is okay in the summer. Keep in mind that pruning during the growing season removes leaf surface that is manufacturing food for next year’s growth. Light pruning will have little impact on future growth, but heavy pruning may slow or dwarf your tree’s growth
• Avoid pruning in the spring to prevent damage to delicate young leaves and buds

The City of Portland has a ban on pruning elm trees during the growing season (from April 15 to October 15). Pruning cuts can attract the elm bark beetle, which is the major vector of Dutch elm disease. Click here to learn more about Dutch elm disease.

Hands-on tree pruning classes are offered by Urban Forestry at different times of the year. View the Urban Forestry Events Calendar for upcoming workshops.

Pruning Types

• Low limbing Removing the bottom limbs of a tree in order to establish clearance heights that meet City Code or provide visual clearance. Also called crown raising. Trees should not be raised more than a foot or two during one pruning cycle, and a large number of branches should never be removed at one time.
• Sucker pruning Clipping the suckers that grow from the base of the tree. This can be done at any time of the year and should be done before the suckers get bigger than ¼-1/2 inch.
• Deadwooding Removing dead branches from a tree. This can be done at any time of the year but is not necessary unless the dead branch becomes a hazard.
• Thinning Pruning branches from a tree for aesthetic purposes. Some thinning is done to reduce wind resistance; however, this is not really necessary because trees often need their entire branch structure to support each individual branch. Thinning should be done in the winter, although limited thinning can be done in the spring. No more than 20-25% of the canopy should be removed during one pruning cycle.
• Through-pruning Also called Y-pruning, this practice is performed by utility companies to train trees to grow around high-voltage wires to give the wires at least 10’ of clearance from the tree. Only utility arborists should prune trees around high-voltage wires.
• Topiary Deliberately pruning trees into fanciful shapes. Topiary pruning is usually performed on evergreen tree species.
• Pollarding Frequent pruning of the upper branches of a tree to keep the tree at a consistent height and promote dense small branch growth. Pollarding can have similar consequences as topping and is generally discouraged.
• Topping The destructive and obsolete pruning practice of cutting back large branches to stubs. You should never top trees! Topping is not allowed in the City of Portland and could could lead to fines or mandatory removal and replanting of topped trees. Topping cuts prevent wound closure, leading to decay and weakly attached new branches.

Proper Pruning Cuts

The branch collar is the swollen area where the branch connects to the trunk. A proper pruning cut should be made just outside the branch collar and should not remove or damage the branch collar. This allows for proper closure of the wound.

To prevent tearing of bark and vascular tissue, use the three-cut approach to pruning:

          Cut #1: Make a small undercut a few inches out from the branch collar

          Cut #2: Remove the limb above cut #1

          Cut #3: Remove the stub by cutting just outside the branch collar

From the Tree Pruning Brochure

• Branches should be pruned at the branch collar-NOT at mid-branch. Mid-branch pruning, called tipping or topping depending on branch size, is harmful to trees, promotes the growth of weakly attached epicormic sprouts, and can lead to the death of the branch or the tree!
• Sharp, clean tools make the smoothest cuts. Choose the tool based on the size of the branch. For small branches (under one inch), bypass pruners should be used. Bypass pruners have a curved cutting blade and are good for pruning tree limbs. For slightly larger cuts (up to four inches) a pruning saw may be used. Chainsaws are preferred when pruning branches over four inches and should be used only by qualified individuals. 

Pruning Young  Trees

• What to prune during the first 5 years after the tree has been planted

Pruning Mature Trees

• Mature trees are an asset for the entire community. Proper care will help ensure mature trees will keep providing benefits to homeowners and the community for years to come

Resources