Pruning Trees along Public Streets

If your tree is growing next to a public walkway or street, you will want to keep it pruned for the safety and protection of others.

Trees growing along streets, sidewalks, and other public areas are governed by city, county, and state codes that are created and put into force to protect the general public.  Trees growing in or along public areas can cause safety issues if they are not correctly maintained.  Acting as good citizens, it is important that we do our part to keep our homes and communities safe for others.

While pruning for public safety is essential, it must be done correctly for no other reason than to maintain the long life and health of the plant.  Improper pruning opens up channels for disease and insect infestation and destroys the natural form of the plant.  Perennial plants store their extra food supplies created through the process of photosynthesis not only in the roots but also in the wood of the trunk and branches.  Heavy pruning removes large quantities of wood and energy needed by the plant.  Weakened trees become more susceptible to insects and diseases.  All of these can cause safety issues in their own right.

Normal pruning techniques require the removal of dead, damaged, or misplaced branches in that order.  In so doing, we try to keep a tree or shrub uniform in appearance on all sides of the plant to mimic what the plant would look like if it were growing wild.  We strive to maintain its natural form.   

I would be the first to agree that pruning for safety might require a stretch of these normal rules of pruning.   Growing alongside a street or sidewalk, a tree or shrub might need to be altered on one side to accommodate pedestrian or vehicular traffic.  Hiding a stop sign, the plant may need to be thinned to allow visibility.  Please, even in these situations, there is no call to hack away at the plant willy-nilly and throw out completely the proven rules of pruning. 

Good pruning helps ensure good health.  If we are going to go through the expense of purchasing, planting, and caring for the plant in the first place, why throw all of that good hard cash out the window by massacring the poor thing?  There are ways to do the job without seriously damaging the plants.

Pruning for safety usually means cutting back trees and shrubs that may obscure the vision of drivers, scrape or scratch vehicles as they pass, or force pedestrians out into a street or road.  In these cases, selective cuts must be made to improve visibility and remove the chance of injury or damage.  Let’s review a few basic rules of pruning and how they might apply to situations where public safety might be an issue.

The easiest and best way to prevent problems later in the plant’s life is to begin the training process early.  Making cuts to young plants when the branches are pencil size and smaller is less stressful than cuts made to larger branches.  Allowing the canopy to develop naturally by removing entire branches that are growing in the wrong way early on will maintain the number of leaves necessary to store energy for the plant and at the same time push the growth in directions that will be less offensive to the public traveling nearby.  Then, as the plant matures, it is a simple matter to make small, less invasive, corrective cuts.

When working on mature trees, don’t just hack at the plant.  The most common but least advised way to meet height limits for trees is to simply hack off the branches at the desired height.  This is not a good idea because the stump ends of the branches are susceptible to insects and diseases.

One of the quickest ways to infect a tree with heart rot or provide an entry way for insects is to leave a blunt end of a cut branch.  Blunt ends and stubs, any cut end of a branch upon which one could hang a hat, almost always die back. Dead or dying wood emits an odor that attracts wood decaying insects like flat-headed wood borers and others.  Attracted to dead or dying wood, they enter in for a free lunch and then set up housekeeping, sometimes even penetrating down into healthy wood.  How can we avoid this?  Don’t leave blunt ends and stubs.

The correct procedure for making cuts is to follow the offending branch back to its point of attachment and make the cut there.  Any cut of a branch pencil size or greater should be made in this manner.  If possible try to force the remaining growth upward and out by retaining branches that grow in the proper direction above the mandated height limitations.  Don’t worry about applying a pruning sealant to the cut.  In our dry climate research has shown that they are not needed.

As you make your cuts it is important to remember to preserve the branch collar.  The branch collar is a swelling of tissue at the base of the branch or limb that is full of  actively dividing cells.  By leaving the collar, a pruning wound will cover and seal over faster than if the collar is cut away. 

We often refer to cuts that remove both the collar and the branch as a “flush cut.”  Flush cuts, or cuts that are made flush with the older branch or trunk, are unhealthy, weaken the branch, and should be avoided at all costs.  Where a branch collar cannot be identified, we look for the branch bark ridge, a ridge of bark between the two branches and then from the base line of that ridge estimate out a 45 degree angle and make our cut.  A cut at a 45 degree angle to the bark ridge should avoid a flush cut and not leave a stub.

So, let’s say that we have a fully mature tree with branches clearly hanging down too far into a roadway.  What do we do?  The first step is to take a good look at the tree and make a plan.  Start by looking at the top of the tree and work down.  Most trees have a central leader, a main trunk from which side branches emerge.  It is important to protect the leader because it will be the point of upward growth for the tree.  Next, study the lateral branches and especially those that hang out into the street.  Identify those branches that absolutely have to come out and those that with a little pruning can be retained and still meet the code limits.  Then, prune all offending branches back to their origin.

With all of this said however, many safety issues needing correction in the landscape can generally be traced back to improper plant selection and placement.  If we have done our homework, we will know that mesquite trees have the capacity to grow up to 30 feet tall and as many feet wide.  Knowing that, we can properly select a planting site where the tree will not later in life cause problems on rights-of-way.  While I hate to say it, a tree or shrub that cannot be trained properly for public safety may just have to be removed.

Human safety is of paramount importance in any landscape.  Correcting any situation that can lead to injury is important and helpful when done in the right way

If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande, Arizona 85122.  The telephone is (520) 836-5221, extension 204. 

The University of Arizona is an equal opportunity, affirmative action institution.  The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85222
Voice:    (520) 836-5221
Fax:    (520) 836-1750
email:    gibsonrd@ag.arizona.edu