The emerald ash borer (EAB) is established in the Denver metro area and actively killing ash trees across the city. If you have an ash tree on your property, you’re facing a decision: treat it, remove it, or watch it decline. Here’s what Denver residents need to know right now.
How EAB Reached Denver
EAB is a metallic green beetle, originally from Asia, that feeds exclusively on ash trees. The larvae tunnel beneath the bark, cutting off the flow of water and nutrients until the tree starves. Once an ash tree is infested, it typically dies within two to four years if left untreated. The beetle has destroyed tens of millions of ash trees across North America since it was first detected in Michigan in 2002.
Colorado confirmed its first EAB detection in Boulder in 2013. The beetle spread to communities along the Front Range in the years following, and Denver now sits squarely within the active infestation zone. The city has responded with its “Be A Smart Ash” program, which plants free replacement trees of different species and removes or treats ash trees in public rights-of-way with resident participation.
How to Tell If Your Ash Tree Is Infested
EAB damage starts at the top of the tree and works downward, so the earliest signs appear where they’re hardest to spot. Initial damage shows up as thinning in the upper canopy, with branches dying over time. By the time the lower trunk shows obvious problems, the infestation is often well advanced.
The most distinctive sign is D-shaped exit holes in the bark, roughly 1/8 inch wide, left behind when adult beetles emerge in June and July. These holes can be oriented in any direction. If you peel back bark from a dying branch, you’ll find S-shaped or “serpentine” tunnels about 1/8 inch wide, packed with a fine sawdust-like material called frass. Increased woodpecker activity is another red flag, since woodpeckers feed on the larvae and will strip bark to reach them.
A tree losing 30 to 50 percent of its canopy is generally too far gone to save with treatment. If your tree still has a mostly full canopy, treatment is worth considering.
Treatment Options for Denver Homeowners
Four active ingredients are used against EAB in Colorado, and they differ significantly in how they’re applied, how long they last, and whether you can do it yourself.
Emamectin benzoate is the gold standard. It’s injected directly into the trunk using specialized equipment, so it requires a professional arborist. The tradeoff is worth it: a single treatment reliably controls EAB for at least two years, and growing evidence suggests it may remain effective for three years between applications. For a large, healthy ash tree you want to keep long-term, this is the most cost-effective option over time despite the higher per-treatment price.
Imidacloprid is the most accessible option for homeowners. It’s typically applied as a soil drench, mixed with water and poured around the base of the trunk, where the roots absorb it. No drilling, no special equipment. The catch: it only lasts one season, so you’re committing to annual treatments. It works excellently on smaller ash trees (under six inches in trunk diameter) in the first year. Larger trees may need two consecutive years of treatment before they’re fully protected.
Dinotefuran is sprayed directly onto the trunk bark or applied as a soil drench. Like imidacloprid, it provides one season of protection and requires annual retreatment.
Azadirachtin requires professional trunk injection. It typically provides two years of protection early in an infestation, but that drops to one year when local EAB populations peak.
When to Treat on the Front Range
Timing matters because these insecticides need to move through the tree’s vascular system, which only works when the tree is actively growing. Soil-applied treatments like imidacloprid drenches are best applied from mid-April to mid-May in Colorado, or again from early September to mid-October. The tree needs warm soil and active root uptake to pull the chemical in effectively.
Trunk injections of emamectin benzoate have a wider window, from April through September. Most arborists in Denver schedule these treatments in spring or early summer to ensure full protection before beetle emergence peaks in June and July. If you’re planning to treat, booking an arborist early in the season is smart since demand is high across the metro area.
Treat or Remove: Making the Decision
The math depends on your tree’s size, health, and location. A large, healthy ash tree providing significant shade to your home can be worth thousands of dollars in property value and energy savings. Treating it with emamectin benzoate every two to three years often costs less over a decade than removal and replacement, especially for trees with trunk diameters over 20 inches, where removal alone can run $1,500 to $3,000 or more.
Removal makes more sense if the tree is already in steep decline, is poorly positioned, or is one of several ash trees on your property. Replacing all your ash trees with a single species just repeats the mistake that made EAB so devastating in the first place. Denver’s urban forest was heavily planted with ash specifically because the trees are tough and fast-growing, which means entire neighborhoods now face losing most of their canopy at once.
If you choose to remove, consider replacing with a mix of species. Denver’s Be A Smart Ash program offers free replacement trees for residents who participate, giving you an alternative species at no cost while helping diversify the city’s urban canopy.
What Trunk Injection Does to Your Tree
One concern worth knowing about: trunk injections require drilling or coring into the tree, which creates small wounds. These entry points can allow disease-causing fungi to establish, particularly a pathogen that causes Nectria canker. For large, vigorous trees the risk is minimal and the benefit far outweighs it. For smaller trees, soil-applied treatments are generally preferred to avoid unnecessary trunk damage. A good arborist will assess your tree’s size and health before recommending an injection approach.
Public Trees and City Property
If the ash tree sits in the strip between the sidewalk and the street, it’s likely in the public right-of-way and falls under Denver’s forestry management. The city’s Be A Smart Ash program handles removal and treatment of these trees directly, though it requires resident cooperation and sign-up. You should not treat or remove a right-of-way tree on your own without checking with Denver’s forestry department first, as permits and regulations apply to trees in public spaces.
For ash trees fully on your private property, the decision and cost are yours. There are no city mandates requiring treatment or removal of privately owned ash trees, but leaving a dead or dying ash standing creates a safety hazard. Dead ash trees become brittle quickly and can drop large limbs without warning, making timely removal important once you’ve decided not to treat.