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Portland Trees

Call before you cut

phone: (503) 823-TREE (8733)


1900 SW 4th Avenue, Portland, OR 97201

How to Root Prune Properly

Before pruning the roots of any street tree, get a free root pruning permit from Urban Forestry.

Root Health

Tree root systems are very large; they can reach as far away from the trunk as the tree is tall. Tree root systems are also very shallow, with 80% of the roots in the top three feet of soil. The roots are integral to the survival of a tree. They support the tree, absorb water and nutrients and store food. A healthy root system insures a healthy tree, so special consideration needs to be taken when working in the root zone or pruning tree roots.

Urban environments are very tough places for trees to live. When a tree is stressed or in decline, the root system should be the first place to look for the cause of the decline. There are many factors that can stress a tree’s root system, including soil compaction, changes in the amount of soil water, additions or removals of soil, or too much fertilizer. Any of these changes can impact root health. It is important to be aware of the changes in soil conditions in order to keep track of the impact to your tree’s root health. 

Pruning Large Roots

Pruning roots or damaging roots in the root zone can have major consequences for the health of a tree. Pruning roots should only be done by professional arborists or after a consultation with a City arborist. Root pruning permits are free and required for trees in the right-of-way.

Pruning Small Roots

Preventive root pruning should be part of a maintenance plan for young street trees. The roots of young trees are actively growing and seeking out new sources of water and nutrients. As roots grow they travel under sidewalks and can raise the sidewalk as they grow larger. Sidewalks are especially susceptible to root damage because unlike roads or driveways, which are poured to depths of 6+ inches over a base of gravel, sidewalks are only four inches deep and lay directly on top of dirt. When roots and sidewalks compete, roots usually win. Therefore preventative root pruning is recommended to reduce the potential for infrastructure damage.

The best tool for pruning the small roots (less than half an inch in diameter) of trees in the planting strip between the sidewalk and the curb is a sharpened nursery spade with a 13” blade. Thrust the spade into the soil along each sidewalk edge to a depth of 12 inches, separating the root ends to prevent them from grafting back together. This should be done annually starting with first year the tree is planted. To prune any other roots on street trees in the planting strip, a free root pruning permit from Urban Forestry is needed.