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Emerald ash borer in Beaverton: what to do about your ash trees
In August 2026 the Oregon Department of Forestry confirmed emerald ash borer in Beaverton and at Bethany Lake Park. It kills nearly every untreated ash it reaches, usually within a few years. The useful decisions are made before a tree shows symptoms, which for Beaverton means now.
By Harry Fauver, founder of Tree Service Beaverton. Updated September 2026.
What has happened, and why it matters
Emerald ash borer is a small metallic-green beetle from Asia, first found in Michigan in 2002. Its larvae feed under the bark of ash trees and cut off the flow of water and sugars. It has since killed hundreds of millions of ash across North America. In the cities it reached early, almost no untreated ash survived.
Oregon's first detection was in Forest Grove in 2022, the first on the West Coast. The state set a permanent quarantine that now covers Washington, Yamhill, Marion, Clackamas and Multnomah counties. The August 2026 finds in Beaverton and at Bethany Lake Park show the beetle moving east through the Tualatin Valley, as expected.
It matters here because ash is common. Native Oregon ash lines Fanno Creek, Beaverton Creek, Rock Creek and nearly every wet swale in the valley, and ash cultivars were planted widely as street and yard trees from the 1980s on. The beetle also attacks olive trees and white fringetree. Mountain-ash, despite the name, is not a true ash and is not affected.
Is it an ash, and is it under attack?
Four things to look for. The first two tell you whether it is an ash, the second two whether the borer is in it.
Know your ash.
Pick what you are looking at.Opposite branching
On an ash, twigs and buds grow in pairs, directly across from each other. Most trees have alternate branching, where they are staggered. Around here only ash, maples, dogwoods and a few others are opposite, and maples and dogwoods have simple leaves, which settles it.
A compound leaf with 5 to 9 leaflets
Each ash leaf is made up of five to nine leaflets arranged in pairs along a central stalk, with a single leaflet at the tip. Walnut and tree-of-heaven have similar leaves but alternate branching. In winter, look for the clusters of paddle-shaped seeds that hang on female trees.
D-shaped exit holes and S-shaped tunnels
Adult beetles chew their way out through holes about an eighth of an inch across and flat on one side, like a capital D. They are hard to spot and often start high in the tree. Where bark has split or been stripped by woodpeckers, you can see the winding, S-shaped tunnels the larvae made.
Dying from the top down
The canopy thins from the top, because that is where attacks begin. Below the dying part, the tree pushes out clumps of new sprouts from the trunk and main limbs. Pale patches where woodpeckers have flaked off the outer bark, called blonding, are often the first sign you can see from the ground.
Treat it or remove it?
It depends mostly on how much canopy the tree still has.
Pick one and the answer appears here.
Worth protecting, if the tree is worth keeping.
This is the best time to act. A healthy, well-placed ash can be protected with trunk injections before the borer reaches it. Decide tree by tree: a large shade tree in a good spot justifies treatment, while a small or badly placed one may be better replaced on your own schedule.
Still treatable. Decide this season.
Trees with less than about 30 percent canopy loss usually respond to treatment, because enough of the tree's plumbing still works to carry the insecticide. Have the tree assessed now. Next year it may be on the other side of the line.
Usually past saving. Plan the removal.
Once this much of the crown has died, treatment rarely brings the tree back, and what survives is a weakened tree with a lot of dead wood in it. Schedule removal while the wood is still sound enough to climb and rig safely.
Remove it soon, and keep clear of it.
Dead ash dries out and becomes brittle far faster than most trees. Limbs snap without warning and the trunk can break during removal, so these jobs take extra care and sometimes a lift. Keep people, cars and play equipment out from under it until it is down.
Replace it on your own terms.
A young ash, or one under wires or crowding a building, is rarely worth years of treatment. Take it out at a convenient time, have the stump removed, and plant something that isn't an ash. Doing it before the tree is infested keeps the job simple.
We assess ash, tell you honestly which group each tree falls in, and remove the ones that have to go. Treatment itself is done by licensed pesticide applicators, and we can point you to one.
What treatment involves
The treatment with the best record is a trunk injection of emamectin benzoate. A licensed applicator drills small holes around the base of the trunk and injects the product, which the tree carries up into its canopy. One treatment protects for two to three years, and then it has to be repeated for as long as you want to keep the tree.
- Timing. Injections are done in the growing season, from mid-spring to early summer, when the tree is moving water.
- Who can do it. In Oregon, applying pesticides to someone else's trees requires a state license. Soil drenches sold to homeowners are much less reliable on large trees.
- What it can't do. It doesn't repair damage already done. A treated tree with a dead top still has a dead top, which needs pruning out.
- Near water. Along creeks and wetlands, product choice and method are restricted. Ask the applicator about buffers.
Treatment makes most sense for mature, healthy ash that shade a house, anchor a garden or line a street, where replacing the tree's benefits would take decades.
Removal and the quarantine
The state quarantine covers all of Washington County. Inside it, ash, olive and white fringetree material, and hardwood firewood of any species, can't be moved out of the quarantine area. For a homeowner that means three things.
- Wood and chips from a removed ash stay on your property or go to a disposal site inside the quarantine area. We handle that as part of the job.
- Don't take firewood from home to a campsite, and don't bring any back. Moving firewood is how the beetle jumps tens of miles at a time.
- Buy firewood where you burn it.
Timing matters for removal as well. An ash that is removed green behaves like any other tree. An ash that has been dead for two or three years is brittle, unpredictable and more dangerous for the crew, which makes the work slower. The park district has already begun taking out ash along some of its trails for that reason.
What to plant instead
The lesson of ash is not to plant too much of any one thing. Pick a replacement suited to the space and different from what your neighbors have. For wet ground where Oregon ash grew, red alder, Pacific willow, black hawthorn and Oregon white oak on the drier edge are native choices. For street trees inside the city, the replacement has to come from Beaverton's approved list.
Related services and guides
Questions people ask
Is emerald ash borer in Beaverton?
Yes. The Oregon Department of Forestry confirmed it in Beaverton and at Bethany Lake Park in August 2026. It was first found in Oregon in Forest Grove in 2022. All of Washington County is inside the state's permanent quarantine.
How do I know if my tree is an ash?
Look for opposite branching, where twigs grow in pairs across from each other, and compound leaves with five to nine leaflets. Female trees carry clusters of paddle-shaped seeds. Mountain-ash is not a true ash and isn't affected.
Can an ash tree be saved from emerald ash borer?
Yes, if treatment starts while the tree is healthy or has lost less than about 30 percent of its canopy. Trunk injections by a licensed applicator protect it for two to three years at a time and have to be repeated.
Should I remove my ash before it gets infested?
Only if it isn't worth treating: a small tree, a poorly placed one, or one already in decline. A large healthy ash in a good spot is usually worth protecting. Whatever you decide, deciding early keeps your options open.
Can I keep or give away the firewood from my ash?
You can keep and burn it on your own property. It must not leave the quarantine area, and neither may any other hardwood firewood. Don't take it camping and don't give it to someone outside the quarantine counties.
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