Street trees receive several types of pruning, the authority and responsibility for which lie in different quarters.
City Activities – Structural Pruning
In the first decade or so after planting, the City conducts what might be termed “pediatric pruning” — circling past every 2-3 years to make the cuts necessary to achieve both the clearance needed at maturity and establish scaffold limbs with good angles of attachment and spacing. Trees that receive this type of training early in life are less apt to have issues with limb failure due to crowding or failure during storms later in life.
After the training phase of life, pruning shifts to a 6-year (approx.) maintenance cycle. This allows timely edits as the canopy expands and matures. Research indicates that a 5-7 year cycle is economically justified. If deferred longer, storm damage increases, and the associated cleanup costs not only outweigh the maintenance pruning costs, but trees endure the kind of damage that can shorten service life.
Most City pruning is done during the winter months when crews are not managing snow and ice.
Utility Company “Line Clearance” Work
Setting the stage: Trees planted under utility lines will NEVER look the same as those growing across the street where there are no lines and certainly not like those growing in the landscape. Once you accept that, then the question to ask yourself is, “Is the tree functional? No matter how forlorn it looks in winter, does it cast shade in summer, detain stormwater, clean the air, provide habitat for insects, birds, mammals, and so on?” If you can get your thinking to that point, then you’re ready to read further…
Compared to a wooden fence post, live wood is a pretty effective conductor of electricity (that’s part of why you shouldn’t stand under a tree during a storm). When limbs are too close to live wires, the current will arc or jump to the tree. During storms, swaying limbs and lines increase arcing, and wet surfaces increase conductivity.
Utility companies are responsible for preventing outages due to line contact with trees. In the late 1990s, they began working closely with professional and research arborists on techniques like “directional pruning” to achieve this goal without doing unnecessary damage to trees with canopies that embrace the sky at utility levels. Due to the considerable risk of working in proximity to live wires, this work is done by specially trained crews contracted to and operating under the direction of a regional arborist with the local utility.
In the planning prior to a pruning cycle, our Public Works Director and the City Arborist work with Duke’s regional arborist to ensure contractors don’t exceed the work necessary for the utility to attain/remain in compliance with vegetation management regulations. We understand that shade conserved is power conserved.
Nevertheless, with increasing constraints on the industry and ever-increasing demand for current, no one should expect trees growing directly under utility lines to look like those growing on the side of the street without utility poles. While there’s no doubt tree appearance immediately after summer clearance work is dismaying, foliage regrows, shade and function return, and the look softens.
Electrical utility hazard training — ~250 words
Utility line clearance certification — ~350 words
Why does utility clearance seem more aggressive or radical in recent years?
The higher the voltage carried in the line, the greater the clearance required. As demand in the region increases, higher-voltage lines intrude farther into your neighborhood and trees have to be trimmed farther back.
Based on voltage and destination, electrical lines on poles are described as transmission, distribution, or drop. None of these lines are jacketed — i.e., unlike the power cord to your microwave, they are not insulated, so current can jump from the line to a grounded object. This can trigger anything from a voltage surge in the home to burns/fires in a tree to outages.
The higher the voltage, the greater distance a spark can jump or “arc”. Transmission lines carry the greatest voltage, 44,000 to 525,000 volts. These are the lines coming out of electrical substations like those in Hartwell and Finneytown that serve Wyoming. Typically, transmission lines don’t run deep into neighborhoods, but as demand increases, so does the intrusion as is clear along Fleming Road from Winton Road nearly to Morts Pass. Distribution lines carry less voltage, 12,000 to 35,000 volts, and are what are normally seen running through a neighborhood. Drop lines are those that emerge from the base of a transformer on a pole and run directly to your home.
The risk of contact between lines and limbs increases with sagging, swaying, and moisture. Lines sag due to extreme heat (whether from temperature or electrical load) and under ice loading. Limbs sag due to lush growth, foliage/fruit weight, water retained during rainfall, and ice-loading. Swaying and moisture increase in storms. So, guidelines for clearance cannot be based on what we see under fair weather conditions.
The historic August 14, 2003 blackout in the Northeast and Midwestern US and Ontario, Canada originated in Northeast Ohio as the result of a transmission line sagging so much due to ambient temperature and current load that it came into contact with a tree canopy in the utility corridor. The cascade of events that followed impacted ~55 million people for periods ranging from a few hours to several days. Check out the Wikipedia page for more information.
The long-term impact of this blackout emerged in Federal policy and regulations related to service reliability. Enforcement is based on fines that can be levied on the utility companies for failure to maintain service if an outage results from a foreseeable/preventable situation. Check out the Wikipedia page for more information.
The regulations began to go into effect ~2006/ 2007. At this writing in 2018, this means most areas have seen 2-3 pruning cycles aimed at more aggressive clearance. There’s some indication that to meet and maintain the new reliability standards, pruning cycles are being pushed to 4-5 years from 5-6 years. At the residential level, the net effect is that utility clearance work is more noticeable, but so too is the reduction in outages.
Duke Energy – Overview of Power Supply — a few short text panels; several pictures.