How Hard Is the ISA Certified Arborist Exam?
Short answer: it is broad more than it is tricky. You answer 200 multiple-choice questions in 3.5 hours across ten domains, and you need 76% of the 180 scored questions, about 137 correct. ISA does not publish a pass rate.
Sources: ISA's Program Guide and exam outline, checked October 19, 2026.
What makes it hard
- Breadth. Ten domains, from tree biology and soil to safe work and urban forestry. Most candidates work in one or two of these every day and rarely touch the others.
- Pace. 210 minutes for 200 questions is about 63 seconds each, with four answer choices and only one correct.
- A high bar. 76% is the passing score, and 20 questions are unscored trial questions you can't identify, so every question has to be treated as if it counts.
- Weighted domains. Your result is weighted by domain, so a weak spot in a heavy domain such as Safe Work Practices (15%) or Pruning (14%) costs more than one in a light domain.
- No published pass rate. ISA doesn't publish one, so there is no official yardstick for how many candidates pass.
On the other hand, eligibility already requires about three years of tree care experience (or a mix of education and experience), so most candidates start with a lot of practical knowledge. The gap is usually the textbook vocabulary and the domains they don't work in.
Where the questions come from
Weights come from ISA's exam outline; the question counts are approximate.
| Domain | Share of exam | ≈ Questions | Free practice |
|---|---|---|---|
| Tree Biology | 11% | 22 | 10 questions |
| Tree Identification and Selection | 9% | 18 | 10 questions |
| Soil Management | 7% | 14 | 10 questions |
| Installation and Establishment | 9% | 18 | 10 questions |
| Pruning | 14% | 28 | 10 questions |
| Diagnosis and Treatment | 9% | 18 | 10 questions |
| Trees and Construction | 9% | 18 | 10 questions |
| Tree Risk | 11% | 22 | 10 questions |
| Safe Work Practices | 15% | 30 | 10 questions |
| Urban Forestry | 6% | 12 | 10 questions |
Weights from the ISA Certified Arborist Program Guide (exam outline based on the Job Task Analysis 2022). "≈ Questions" is the weight applied to 200 questions; ISA does not publish exact per-domain counts.
Try 5 exam-style questions
One medium-difficulty question from each of the 5 heaviest domains. Pick an answer to see the explanation and its source.
A ground worker controlling a lowering line through a friction device at the base of the tree finds that pieces are coming down too fast. Which change most directly adds friction?
Show answer and explanation
Correct answer: D. Adding another wrap of the line around the device
A lowering device works like a capstan: the friction it produces rises with the angle of rope wrapped around it, so each added wrap gives the ground worker more holding power for the same pull on the tail. Taking a wrap off does the opposite. A pulley at the rigging point reduces friction aloft rather than adding it, and a longer rope changes how much energy the system absorbs, not the friction at the device.
- A: A pulley is designed to reduce friction at the rigging point, which leaves more of the load for the ground worker to hold.
- B: Rope length affects how much energy the system absorbs, not how much friction the lowering device creates.
- C: Removing a wrap reduces the wrap angle and therefore the friction.
Sources: Dynamic parameters of lowering loads at gradual tree felling (Journal of Forest Science 71(2): 86-98, 2025), Czech Academy of Agricultural Sciences ; Dynamic parameters of lowering loads at gradual tree felling (Journal of Forest Science 71(2): 86-98, 2025), Czech Academy of Agricultural Sciences
Report a problem with this question · ID safe-021
An arborist is making the final cut to remove a live branch that has a visible branch bark ridge and branch collar. Where should the cut begin and end?
Show answer and explanation
Correct answer: B. Begin outside the branch bark ridge and angle down and out to just past the collar
Clemson Extension describes the final cut as starting outside the branch bark ridge and ending just outside the branch collar swelling on the underside, angled down and away from the trunk. This removes only branch tissue and leaves the collar, which helps wall off decay, intact. A flush cut or a cut into the collar injures trunk tissue, while cutting farther out leaves a stub that delays wound closure.
- A: A flush cut removes the collar and injures trunk tissue, inviting decay.
- C: Cutting well out on the branch leaves a stub that delays closure.
- D: Cutting into the collar removes the tissue that helps resist decay.
Source: Principles & Practices for Pruning Trees (HGIC 1003), Clemson Cooperative Extension, Home & Garden Information Center
Report a problem with this question · ID prun-012
Discoloration and decay behind a trunk wound usually spread much farther up and down the stem than inward or around it. In the CODIT model, which wall resists that vertical spread?
Show answer and explanation
Correct answer: A. Wall 1, the plugged vertical conducting cells
Wall 1 forms when the vessels or tracheids above and below a wound plug, completing the tops and bottoms of the compartments. It is the weakest of the four walls, which is why columns of discolored and decayed wood usually extend farthest vertically. Wall 2 resists inward spread, Wall 3 (the rays) resists spread around the trunk, and Wall 4, the barrier zone, is the strongest.
- B: Wall 2 resists inward spread toward the pith and is the second-weakest wall.
- C: Wall 3, the rays, resists lateral spread around the trunk and is fairly strong.
- D: Wall 4 is the strongest wall and separates old wood from wood formed after wounding.
Source: Compartmentalization of Decay in Trees (Agriculture Information Bulletin 405), USDA Forest Service
Report a problem with this question · ID bio-012
During street tree inspections, an inspector finds a vigorous, structurally sound maple whose low limbs hide a stop sign from approaching drivers. How should this be handled in a community tree risk program?
Show answer and explanation
Correct answer: D. Treat it as a public-safety risk and prune the limbs for clearance
Tree risk is not limited to structural failure. The USDA Forest Service guide counts trees that visually block intersections, traffic signs or street lighting among the public-safety risks a program should reduce, and it directs crews to remove branches that obstruct signs, signals or lighting or that limit drivers' view of approaching traffic. A sound tree can still create a serious risk by hiding a stop sign, so calling it no risk or waiting for decay misses the problem.
- A: A tree can create a traffic risk without any structural defect.
- B: Clearance pruning of trees that block signs is part of the tree risk program's work.
- C: The risk comes from the blocked view now, not from future decay.
Sources: Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry ; Urban Tree Risk Management: A Community Guide to Program Design and Implementation (NA-TP-03-03), USDA Forest Service, Northeastern Area State and Private Forestry
Report a problem with this question · ID risk-009
Ginkgo (Ginkgo biloba) has broad, fan-shaped leaves, yet it is classified as a gymnosperm. Which characteristic places it in that group?
Show answer and explanation
Correct answer: B. Its seeds are not enclosed in a fruit (ripened ovary)
Gymnosperms produce seeds that are not contained in a ripened ovary, while angiosperms enclose their seeds in a fruit. The smelly 'fruit' of a female ginkgo is really a seed with a fleshy covering, so ginkgo is a gymnosperm despite its broad leaves. The other choices are true of ginkgo, but being deciduous or having separate male and female trees also occurs in many angiosperms, so those traits do not define the group.
- A: Ginkgo is deciduous, but so are many angiosperms; leaf drop does not define gymnosperms.
- C: Separate male and female trees also occur in angiosperms such as hollies, so this does not define gymnosperms.
- D: Vein pattern is a useful identification feature, but it is not what separates gymnosperms from angiosperms.
Sources: Extension Gardener Handbook, Chapter 3: Botany, NC State Extension ; Ginkgo, The Morton Arboretum
Report a problem with this question · ID ids-006
0 of 5 correct
How to make it easier
- Read ISA's free exam outline and mark the domains you never deal with at work.
- Start with the heaviest domains and your weakest ones, using practice questions by domain.
- Review every miss: why the right answer is right and why yours was wrong.
- Take at least two full timed practice exams before exam day to get used to the pace.
Our free study guide turns this into an 8-week plan, and the exam guide covers eligibility, scheduling and retakes.
Questions about the difficulty
What is the pass rate for the ISA Certified Arborist exam?
ISA does not publish one in its Program Guide, and we have not found an official figure (checked October 19, 2026). Pass rates quoted elsewhere without a source should be treated with caution.
How many questions can I get wrong?
20 of the 200 questions are unscored trial questions, and you can't tell which. On the 180 scored questions, 76% means about 137 correct answers. Treat every question as if it counts.
Is there enough time?
You get 210 minutes for 200 questions, about 63 seconds each. That is enough for a prepared candidate, but not for working out every answer from scratch, so timed practice matters.
How long should I study?
ISA does not set a study time. Our free study guide suggests an 8-week plan for someone who already works in tree care, with more time for domains you rarely meet in your work.
Official sources
- ISA Certified Arborist Program Guide (revised August 2025) (format, pretest questions, passing score)
- ISA Certified Arborist Examination Outline (based on Job Task Analysis 2022) (domains and weights)
We are not affiliated with ISA. This page summarizes ISA's published rules in our own words; if anything here differs from ISA's current documents, ISA's documents win.