Has the emerald ash borer (EAB) arrived in Denver?
Yes. After moving across North America and parts of Canada, the Emerald Ash Borer (EAB) first showed up in our region in Boulder in 2013, in Arvada in 2021, in Littleton in 2023, Lakewood in 2024 and Denver in 2025.
How much of Denver’s urban forest consists of ash trees?
EAB has the potential to destroy Metro Denver’s 1.45 million ash trees (330,000, or 1 in 6, trees in the City and County of Denver, and 1 in 10 on public land, including parks, right-of-ways and boulevards). Due to their fast growth and high aesthetic appeal, ash trees have been widely planted in Denver, including on private property, public parks and other community areas.
Why should I care about Denver’s tree canopy?
Trees are critically important to our overall quality of life. Among other things, they produce oxygen, reduce smog, cool our neighborhoods and homes, and increase our property values. One in six trees in Denver are ash trees. On public land, including right-of-ways, boulevards and parks, 1 in 10 trees in Denver are ash trees. And if we do nothing, it’s just a matter of time before they’ll ALL be devoured by the emerald ash borer.
How can I identify an ash tree?
Images and information about identifying ash trees are available at BeASmartAsh.org.
Denver residents can determine if a tree is an ash by looking at the branch and bud arrangement, leaves, bark and seeds. Ash is unique from other trees because of its compound leaf structure and opposite branching. A compound leaf is one that has more than one leaflet while the entire leaf has a bud at its stem base (petiole). Ash trees typically have 5-9 leaflets per leaf.
Opposite branching trees have side branches growing directly opposite of each other. There are far fewer trees with opposite branching than those with alternate branching.
On mature ash trees, the bark is tight with a distinct pattern of diamond-shaped ridges. Conversely, on young ash trees, the bark is smooth. Seeds, which are dry, oar-shaped samaras, typically hang on the tree until late fall or early winter.
A photo gallery is available at BeASmartAsh.org.
What does the EAB look like?
EAB is an attractive insect with a dark, metallic-green body and a coppery red or purple abdomen under its wings. To give you some perspective, at approximately a half inch long, an adult EAB can fit comfortably on a penny.
Are there other insects that are similar in appearance to the EAB?
There are several insects native to Colorado that one could easily mistake for EAB. There are even some other types of insects that tunnel into ash trees. That’s why Be a Smart Ash is about more than just being informed; it’s about being aware of what you don’t know. If you are unsure about the possible presence of EAB in or on an ash tree, call Denver’s Office of the City Forester, an arborist or other tree care professional.
What are signs that an ash has been attacked by the EAB?
It can take 2-4 years for impacted trees to show signs of infestation, but watching the surrounding areas may alert you to the presence of EAB. There are several telltale signs of EAB infestation. Look for yellowing and dieback in the upper canopy, thinning or dying sections, as well as increased bird activity, such as more Northern Flickers, a type of large brown woodpecker. Eventually, S-shaped tunnels can be seen on the trunk and tiny, D-shaped exit holes are often visible. Additionally, impacted trees show signs of bark shedding. It can take 2-4 years for impacted trees to show signs of infestation.
Can I keep my ash tree?
Denver residents who want to keep their ash trees should work with a licensed and insured arborist or tree service company to determine if a treatment plan or removal and replacement is the best approach. When it’s appropriate and administered correctly, treatment can be 90% effective and must be repeated every 2-3 years. Choose from this list of approved tree services and get information about state licensing.
Are there other options besides treatment?
Given the serious threat posed by EAB, removing and replacing ash trees is a viable option. In fact, depending on the size, condition and location of the tree, replacement may be the smartest choice. Dead or alive, ash trees can be safely removed by a licensed and insured arborist or tree service company. Denver residents can also apply for a free replacement tree.
Is ash still a viable choice when considering what to plant in my yard?
The short answer is no. Further planting of ash trees in Colorado is not recommended. The planting of diverse tree species in your yard or in your community is highly encouraged, as this is our urban canopy’s best defense against most tree health problems. The Office of the City Forester has created a list of approved trees for the right-of-way, but it can be useful in selecting a tree for your yard.
How does such a small insect harm or destroy ash trees?
In short, they kill ash trees by disrupting the flow of essential water and nutrients. The adult EAB lands on the tree and lays eggs. The eggs hatch and become worm-like larvae that tunnel through the tree’s water-conducting tissue just under the bark. The tunneling and feeding under the bark is what eventually kills the impacted tree. It can take 2-4 years for an impacted tree to show signs of infection, which can give tree owners ample time to battle EAB.
Will Denver Parks & Recreation/The Office of the City Forester remove, treat or pay to replace ash trees on my property?
No. Residents or property owners are expected to take responsibility for any ash trees on their property.
What is the City of Denver doing to combat this problem?
The Office of the City Forester, part of Denver Parks & Recreation, is in it for the long haul and is 100% committed to battling EAB. It took the first, big step by creating the Be A Smart Ash campaign (BeASmartAsh.org) in 2016 and addressing preventative options for public trees.. Through the educational campaign, the City Forester has turned Denver residents into Smart Ashes who can identify, treat and/or replace their ash trees.
Additionally, the City has replaced many small ash trees on City-maintained land, including parks. However, some of the historic ash trees on city property are irreplaceable. For those trees, the City has a rigorous treatment schedule.
The Office of the City Forester has also offered free replacement trees to Denver residents. Since 2016, the Office of the City Forester has planted 33,670 trees.
How can I become a Smart Ash?
- Identify ash trees, regularly inspect for signs of EAB and contact the Office of the City Forester immediately at forestry@denvergov.org or 720.913.0651 if ash trees display any symptoms of EAB infestation.
- Talk to neighbors, friends and co-workers about EAB. Spread the word and encourage people to visit BeASmartAsh.org for more information.
- Do not transport firewood or other products from ash trees, as EAB larvae stealthily survive and travel hidden under the bark. If an ash tree absolutely must be moved, the wood must be chipped smaller than one inch.