Written by Justin Tornatore, ISA Certified Arborist | TIMBERx
Abstract
Anyone with a truck and a chainsaw can knock on your door and offer to "trim" your trees. Hiring tree work is still one of the few home decisions where a bad job can take years to show up and decades to fix. The International Society of Arboriculture (ISA) describes its arborist certification as a voluntary, nongovernmental credential earned by passing a comprehensive exam, maintained through continuing education, and bound by a Code of Ethics [1]. This article explains what that credential does and doesn't mean, reviews what ISA and University of Arizona Cooperative Extension say about harmful practices like topping [2][3], and gives Tucson homeowners a practical checklist for hiring tree care, whoever they hire.
Background: What Is an Arborist, Exactly?
An arborist is a specialist in the care of individual trees. That covers pruning, diagnosis, planting, risk, and removal. "Tree trimmer," "tree guy," and "landscaper" aren't credentials. They're job descriptions, and the knowledge behind them ranges from excellent to nonexistent.
The ISA Certified Arborist credential gives homeowners a measurable baseline. According to ISA, certification "provides a measurable assessment of an individual's knowledge" in tree biology, diagnosis, maintenance practices, safety, and related areas. Certified arborists must continue their education to keep the credential, so they're "more likely to be up to date on the latest techniques" [1]. ISA certification holders must also follow a Code of Ethics and use industry-accepted practices and standards [1].
I'll be honest about the limits too. A certificate is a floor, not a guarantee. It tells you a person has demonstrated a base of knowledge. It doesn't replace references, a clear written scope, or your own eyes on the work.
What the Research and Standards Show
Topping is the clearest dividing line. ISA calls topping "perhaps the most harmful tree pruning practice known." It defines topping as cutting branches to stubs or to laterals too small to take over as the leader [2]. Here's what ISA says topping does [2]:
- It can remove 50 to 100% of a tree's leaf-bearing crown, which can starve the tree.
- It forces rapid growth of multiple shoots below each cut, as much as 20 feet in one year in some species. Those shoots are anchored only in the outer layers of the branch and are weakly attached.
- It exposes remaining bark to sunburn and leaves wounds the tree may not be able to close, opening a path for decay.
- The result is that a tree topped to "reduce risk" often becomes more likely to drop limbs.
UA Cooperative Extension agrees. Its pruning guidance calls topping "extremely detrimental to any tree species" and recommends leaving the removal of heavy, large limbs to professionals [3].
Good pruning follows standards and limits. UA's palm publication points readers to the ANSI pruning standards available through ISA, and it lists the core rules: define a clear reason for each cut, prune at the correct time, and follow correct practices [4]. Pima County Extension guidance caps removal at one quarter of a tree's canopy at one time [7].
Climbing spikes have a place, and pruning isn't it. UA warns that no nails or spikes should be used to climb palms during pruning, because palm wounds are permanent [4].
Storm work is dangerous work. ISA warns that trees and limbs under tension "may react unexpectedly when cut or moved," and that work near electrical lines requires specialized knowledge [5].
ISA's hiring checklist [1] recommends that homeowners:
- Check for an ISA credential, and verify it using the "Verify a Credential" tool on TreesAreGood.org.
- Check for membership in professional organizations such as ISA, the Tree Care Industry Association, or the American Society of Consulting Arborists.
- Get more than one estimate, and ask for references.
- Don't automatically accept the lowest bid. Compare credentials and written specifications.
- Be wary of people who go door to door offering bargains.
- Get it in writing, and read the contract carefully.
Field Observations
The giveaways of poor tree work are visible from the street once you know what to look for. Stubs left sticking out from the trunk. Flush cuts that remove the branch collar. "Lion-tailed" limbs stripped of everything but a tuft at the end. Palms cut to a narrow "pineapple." Mesquites and palo verdes rounded into lollipops. In my pruning article, I covered why topping, flush cuts, and "lions tails and lollipops" hurt a tree. They remove too much leaf area, leave wounds the tree can't conceal, or push weight out to the ends of branches.
The best way I can put it is this: a good arborist should be able to explain why every cut is being made. If the answer is "because it looks cleaner" or "because that's how we always do it," keep looking.
Practical Recommendations
Questions to ask any tree care provider, including us:
- "Are you ISA Certified, and what's your certification number?" Then verify it on TreesAreGood.org [1][6].
- "What's the goal of this pruning?" Look for answers like removing deadwood, fixing structure, clearing a roof, or storm prep, not "a nice clean look."
- "How much of the canopy will you remove?" Anything approaching or exceeding a quarter of the canopy deserves a second opinion [7].
- "Where will you make the cuts?" Listen for "at the branch collar" or "back to a lateral branch," not "we'll take the top down."
- "Will you top the tree?" The only right answer is no [2][3].
- "Will you use climbing spikes?" For pruning, especially palms, the answer should be no [4].
- "Can I get the scope in writing?" [1]
Watch for these red flags:
- Door-to-door offers, especially right after a storm [1].
- Pressure to decide on the spot.
- Promises to "make the tree safe" by topping it [2].
Key Takeaways
- ISA certification is voluntary, exam-based, and requires continuing education and a Code of Ethics. You can verify it online [1].
- Topping harms trees and increases future risk, according to ISA and UA [2][3].
- Good pruning has a reason for every cut, follows ANSI standards, and stays under a quarter of the canopy [4][7].
- Ask questions, get it in writing, and don't let the lowest bid make the decision for you [1].
Bottom line from your local arborist: your trees will outlive most of the people who prune them. Hire someone who prunes like they know that.
Talk to an Arborist
Have questions about a quote or a pruning plan? Text Justin at (520) 451-8790 (texting is preferred over calling) or email Justin@TimberxTrees.com. TIMBERx is a veteran-owned tree care company led by Justin Tornatore, ISA Certified Arborist, serving Tucson, Oro Valley, Marana, Vail, Sahuarita, Green Valley, and Benson.
References
- International Society of Arboriculture. (2021). Why Hire an Arborist? TreesAreGood.org. https://www.treesaregood.org/Portals/0/TreesAreGood_Why%20Hire%20An%20Arborist_0321.pdf
- International Society of Arboriculture. (2021). Why Topping Hurts Trees. TreesAreGood.org. https://www.treesaregood.org/Portals/0/siteContent/pdf/Web%20TAG%20brochures/TAG_CBweb_Why%20Topping%20Hurts_v2.pdf
- Davison, E. (2015). Pruning Deciduous Shade Trees (AZ1139). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/pruning-deciduous-shade-trees
- Schuch, U., & Quist, T. (2023). Arizona Landscape Palms and their Management (AZ2021). University of Arizona Cooperative Extension. https://extension.arizona.edu/publication/arizona-landscape-palms-and-their-management
- International Society of Arboriculture. (2021). Storm-Related Tree Damage. TreesAreGood.org. https://www.treesaregood.org/Portals/0/TreesAreGood_Storm%20Related%20Tree%20Damage_0721.pdf
- International Society of Arboriculture. Find an Arborist / Verify a Credential. TreesAreGood.org (n.d.; accessed October 2026). https://www.treesaregood.org/Find-an-Arborist
- Larson, P., Vance, L., & North, D. (2025). July Monthly Gardening Guide for Pima County. University of Arizona Cooperative Extension. https://www.extension.arizona.edu/publication/july-monthly-gardening-guide-pima-county
