What does an arborist do? Roles, credentials and when you need one in WA or OR

What is an arborist? The plain meaning

Federal job data define the work in practical terms. The Bureau of Labor Statistics files arborists under tree trimmers and pruners, “also called arborists,” who cut away dead or excess branches to clear utility lines, roads and sidewalks; some specialize in diagnosing and treating tree diseases, others in pruning and shaping ornamental trees. O*NET, the Labor Department’s occupation database, lists Arborist among the job titles reported for occupation 37-3013, along with Climber, Groundsman, Plant Health Care Technician and Tree Trimmer.

The International Society of Arboriculture (ISA) gives the fuller list of what arborists provide: pruning, removal, storm and emergency work, planting advice, plant health care, cabling and bracing, soil aeration, lightning protection, tree risk assessment, tree protection during construction, and consulting and legal services. How to find one, check one and pay one is covered on the arborist services hub for Washington and Oregon; this page sorts out the roles and titles you will run into.

What does an arborist do? Five kinds of arborist work

Arborist work splits into five lines, and most people spend their careers in one or two. The table pairs each with the credential that fits it and who usually pays for it.

Sources: O*NET 37-3013 tasks and job titles; ISA credential list and TreesAreGood brochures; ASCA; WAC 296-45-035; RCW 17.21.070; Oregon Department of Agriculture. The “who hires them” column is this site’s summary. Checked 2026-10-02.

Most of these jobs are with tree and landscape companies. In May 2025 BLS counted 55,160 tree trimmer and pruner jobs nationally, and 49,410 of them, 89.6%, were in landscaping services, the industry whose index entries include arborist services, tree removal and tree trimming. Local governments held 2,650 and utilities 1,040. BLS’s regular estimates give those industry splits only for the nation, not for Washington or Oregon.

O*NET’s task list for production work runs from feeding the chipper and lowering limbs on ropes to climbing, cabling and bracing, and training newer workers. In its survey of the occupation, 48% of workers said they work at heights every day.

The other four roles are narrower. Utility crews work on a schedule: Portland General Electric says its contract tree crews inspect and prune along its lines every two to three years. Plant health care, as ISA describes it, is routine monitoring and preventive treatment. Applying pesticides to someone else’s property as a business takes a state license: a commercial pesticide applicator license in Washington (RCW 17.21.070), and in Oregon the Department of Agriculture’s commercial operator and applicator licenses, where tree and shrub work falls in the Ornamental and Turf category.

Certified arborist credentials, and how to check them

ISA’s credentials are the ones Northwest cities write into their codes. They are voluntary and run by a nongovernmental body; ISA’s own program guides say its certifications “do not represent licensure.” The ones you will see on a bid or a report:

To check a person, ask for the certification number and look it up in ISA’s online credential directory, linked from TreesAreGood.org. Cities lean on these credentials. Portland’s tree code defines an arborist as an ISA Certified Arborist or a consulting arborist registered with ASCA (PCC 11.80.020). Bellevue accepts an ISA certified arborist, municipal specialist or Board Certified Master Arborist, an ASCA Registered Consulting Arborist or a Society of American Foresters certified forester as a qualified tree professional. Spokane requires an ISA Certified Arborist holding the city’s Commercial Tree License for any pruning, planting or removal of public trees.

Tip: certification is personal, not corporate. “ISA certified” on a company’s website means someone there holds a number; ask whose it is and whether that person will be the one looking at your tree. The full path to the credential is in how to become an arborist.

Arborist vs tree service: who does what

An arborist is a person; a tree service is a business. A certified arborist can work for a tree service, run one, or never cut a tree, and a tree service can employ certified arborists or none. The state rules attach to the business. This is general information, not legal advice; the rules are paraphrased from the codes and agency pages in Sources.

The “tree guy” with a saw and a pickup is a tree service without the paperwork. Tree work for hire takes the same paperwork as anyone else’s: Oregon’s law covers removing or pruning trees, Washington’s names tree removal, and Washington registration puts a bond on file ($15,000 for a specialty contractor) along with liability insurance of at least $200,000 public liability and $50,000 property damage, or a $250,000 combined limit. Oregon’s Landscape Contractors Board also offers an arborist license phase to ISA Certified Arborists. Check any business on Washington tree service license lookup or Oregon tree service license lookup.

Seattle draws the line most clearly. Under SDCI Tip 242C, commercial tree work includes removing a tree 6 inches or more in diameter, cutting live branches 4 inches or larger, and even assessing a tree’s health or hazard risk (SMC 25.11.130). Only registered providers may do it, and registration takes an ISA Certified Arborist on staff or retainer, TRAQ for hazard assessments and $1,000,000 in liability insurance; unregistered work can draw double penalties. Which cities want a written report, and who may sign it, is in the arborist report guide.

When do you need an arborist, not just a tree crew?

Hire for the question you have. A crew answers “how do we get this done safely”; an arborist answers “should this be done at all,” and puts it in writing when someone else needs to see it.

• A permit or exemption that needs paperwork. Hazard-tree removals in Seattle, Kirkland and Shoreline turn on an arborist’s written risk assessment, Lake Oswego can require one, and Portland asks for photos or other documentation, such as a report by a certified arborist, after an emergency removal. The city rules are on Washington tree removal permits and Oregon tree removal permits.
• A risk question. A crack, a new lean or a big limb over the bedroom: a TRAQ-qualified tree risk assessment rates the likelihood of failure, the likelihood of impact and the consequences.
• A diagnosis. ISA lists discolored leaves, dead branches and early leaf drop as reasons to call; the fix may be a change in watering rather than a saw.
• Construction near trees. ISA advises bringing an ISA Certified Arborist in at the planning stage to set a tree protection zone, because root damage from grading and trenching can take years to show.
• An appraisal. When a storm, a car or a neighbor’s crew destroys a tree, ISA notes that appraisal guidelines are recognized by insurers, courts and in some cases the IRS, and that the appraisal should be made soon after the loss.
• A weak fork or a heavy limb. Supplemental support is arborist work done to ANSI A300; see tree cabling and bracing.

For a plain job, such as pruning a healthy tree or removing one that needs no permit, a licensed tree service with a certified arborist on staff covers both questions; how to find and check one is on the arborist hub. Modeled prices for tree removal and tree trimming are on those hubs.

Safety: if a tree or limb touches or hangs within 10 feet of a power line, call the utility first. Only utility or qualified line-clearance crews work inside that distance (WAC 296-45; OAR 437-002-0306), and a general tree crew is not a line-clearance crew; see utility tree trimming.

How many arborists work in Washington and Oregon?

The closest count is BLS occupation 37-3013, tree trimmers and pruners, in the May 2025 wage survey. It covers people paid a wage or salary and leaves out the self-employed and owners of unincorporated firms, so many owner-operators are missing.

Oregon’s count is more than twice Washington’s, but treat both job counts as rough: BLS puts their relative standard errors at 17.1% and 22.9%. The wages are firmer, and they are what workers earn, not what a customer pays. Pay by metro and percentile is in the arborist salary guide, and how this site turns those wages into modeled crew-hour prices is on the methodology page.