Emerald ash borer is now in Denver, and if you have an ash tree on your property, the window to protect it is shorter than most homeowners think. This guide covers how to confirm you have an ash, the signs of an emerald ash borer infestation, and how to decide between treatment and removal.
Is Emerald Ash Borer in Denver?
Yes. Lab results and a forester’s site visit confirmed emerald ash borer in Denver on June 17, 2025, after inspectors found an infested tree in Congress Park. The beetle first showed up in Boulder in 2013, and the Colorado State Forest Service has since detected it in more than 20 Front Range cities. Denver has about 330,000 ash trees, one in every six trees in the city, which is why the regional Be A Smart Ash campaign has been pushing identification and treatment for years. On its own the beetle only spreads about half a mile a year, but it moves much faster with human help, especially in firewood. An infestation two neighborhoods over can become your problem sooner than you’d expect.
North of Denver, the beetle was already confirmed in Westminster and Broomfield by late 2019, after Boulder County’s 2013 quarantine failed to contain it. Each city forester sets its own rules for ash on private property, so check with yours before you decide.
What Is Emerald Ash Borer and How Does It Kill Ash Trees?
Emerald ash borer is a metallic green beetle native to Asia that lays its eggs on the bark of ash trees. Once the larvae hatch, they tunnel under the bark and feed on the layer that moves water and nutrients through the tree. That damage cuts off the tree’s circulation, and the canopy starts dying from the top down. Since it was discovered in 2002, the insect has killed hundreds of millions of ash trees in North America, and infested trees typically die within three to four years without treatment. It only attacks true ash trees in the genus Fraxinus, so mountain ash isn’t at risk despite the name.
How Do I Know If I Have an Ash Tree?
Check three things: branching, leaves, and bark. Ash trees have opposite branching, meaning two branches grow directly across from each other on the stem instead of alternating. The leaves are compound, usually five to nine leaflets on a single stalk. Mature bark has a tight, diamond-shaped ridge pattern.
- Branches and buds grow directly opposite each other
- Leaves are compound, not single, with 5 to 9 leaflets
- Mature bark shows a diamond ridge pattern
- Seeds, if present, are single paddle-shaped samaras
No single trait confirms it on its own. Check branching first, since that rules out most look-alikes fast, then confirm with leaves and bark.
Signs Your Ash Tree Has Emerald Ash Borer
An early infestation is hard to spot, because most of the damage happens under the bark before anything shows up top. By the time symptoms are obvious, the tree has usually been infested for a year or more. That applies across the metro, from central Denver to western suburbs like Arvada. Watch for:
- Canopy thinning or dieback starting at the top of the tree
- Small D-shaped exit holes, about 1/8 inch wide, in the bark
- Vertical bark splits exposing winding tunnels underneath
- New shoots (epicormic sprouts) growing from the trunk or base
- More woodpecker activity or bark flecking
If you see two or more of these, get a professional evaluation instead of guessing. Our emerald ash borer handbook has more detail on what to look for.
Should You Treat or Remove Your Ash Tree?
This is where most homeowners get stuck, and the answer depends on how much canopy the tree has already lost. Colorado State University Extension and Purdue use the same cutoff: trees that have lost more than 30% of their canopy are poor candidates for treatment, because there isn’t enough healthy tissue left to carry the insecticide through the tree. Below that threshold, the odds are good. If your tree qualifies, emerald ash borer treatment with trunk injections can protect it. If it’s past the point of saving, professional tree removal is the safer option, since dead ash wood gets brittle fast.
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