Quick answer: Treat an ash tree that is healthy, well placed and valuable enough to protect for the rest of its life. Remove one that is already declining, small, badly located or growing under power lines. Treatment protects healthy trees but cannot rescue a failing one, and it has to be repeated for as long as the tree stands.
Emerald ash borer was confirmed in Denver in June 2025, so every ash owner faces this choice. This guide lays out how to make it. For an inspection, see ash tree removal and emerald ash borer help in Denver.
Which trees should be treated and which removed?
| Your ash tree | Usual choice |
|---|---|
| Large, healthy, full crown, good location | Treat |
| Healthy but small | Replace with another species |
| Thinning crown, dieback at the top | Remove |
| Under power lines or crowding a building | Remove |
| Dead or mostly dead | Remove soon |
A widely used rule of thumb is that an ash which has lost more than about a third of its canopy is a poor candidate for treatment.
What should you know about treatment?
- It works on healthy trees. The city’s Be A Smart Ash program puts properly applied treatments at about 90% effective.
- It is for life. Be A Smart Ash points out that an ash needs proactive treatment throughout its life to survive, which is why it recommends replacing smaller ash trees.
- It needs a licensed applicator. The Colorado State Forest Service states that anyone hired to apply pesticide to protect ash trees must be licensed by the Colorado Department of Agriculture as a commercial pesticide applicator. You can check a company on the department’s applicator search.
- Do not be rushed. The Forest Service also advises not to be pressured by door-to-door sales and to get at least three quotes.
What should you know about removal?
Removal is a one-time cost and ends the risk. It is safer and cheaper while the tree is still alive, because ash killed by emerald ash borer turns brittle and is harder to take down. See tree removal cost in Denver. If the ash stands in the public right-of-way, a free permit is needed first: see tree removal permits in Denver.
What about replacing the tree?
Planting a different species now means the new tree is growing while you decide about the old one. See what to plant instead of ash in Denver. Some residents qualify for a free street tree, covered in Denver’s free tree and ash replacement programs.
How do you make the decision?
- Confirm the tree is an ash. See how to identify an ash tree.
- Have its health assessed, especially the upper crown.
- Weigh the cost of treating for decades against one removal and a new tree.
- Act before the tree declines. A failing ash has only one option.
Ash tree treatment and removal FAQs
Is it too late to treat my ash tree?
Not if the tree still has a full, healthy crown. Once an ash has lost a large part of its canopy, treatment is unlikely to save it and removal is the better choice.
How often does an ash tree need to be treated?
Treatment has to be repeated for the life of the tree. The interval depends on the product used, so ask the licensed applicator for the schedule and the cost over several years.
Do you treat ash trees for emerald ash borer?
We inspect ash trees, tell you whether a tree is a candidate for treatment, and remove ash that are declining or badly placed. Insecticide treatment itself must be carried out by a state-licensed pesticide applicator.
