Boulder's urban canopy is at risk, and it's going to become even more important as climate change makes things hotter. Courtesy of the City of Boulder

A portion of Boulder’s trees are dying, or soon will be. As Boulder gets hotter with climate change, this is a problem. Because trees cool things down.

About 25% of the urban canopy in the City of Boulder are ash trees, and ash trees are susceptible to a bug that’s systematically killing them. Called the Emerald Ash Borer, the pest is an invasive species from Asia that bores into trees of interest, stunting their ability to deliver nutrients up and down their trunks. 

“It girdles the tree,” said Michelle Ferguson, a forester with the city. 

The ash borer was discovered in Boulder about 10 years ago, though it was likely in town for some time before that. It was first discovered in North America in 2002, in Michigan, but could have been on the continent since the 1980s. It’s suspected to have come from overseas in wooden packing crates. As the climate warms, the ash borer’s potential range is expanding.

Boulder’s foresters are treating a portion of trees across town with trunk injections that Ferguson said are “really effective.” The injections, the most common being Emamectin Benzoate, makes the tree unpalatable to new borers and kills larva already inside. Yet it’s not a one-time deal. The trees have to be treated every two to three years. (The city has a map of infested trees it is treating, along with many it’s not.)

Ferguson said 1,300 trees in parks and public spaces are getting the injections, and have been since the spring of 2014. The city has also done outreach to private property owners, offering to help them treat trees on their land. As for the rest? They’re going to die. 

“Any untreated ash tree will eventually get [Emerald Ash Borer] and need to be removed,” Ferguson said.

An Emerald Ash Borer, the pest responsible for putting 25% of Boulder’s urban canopy at risk. Courtesy of the Pennsylvania Department of Conservation and Natural Resources

According to the city’s website, over the next eight years or so it will remove about 4,500 ash trees from city parks and public spaces, representing a fraction of those infected. The city’s Urban Forest Strategic Plan, put out in 2018, estimated there were 70,000 at-risk ash trees in Boulder, including privately owned trees. (A majority of Boulder’s urban canopy is made up of trees on private land.)

The potential loss of these 70,000 trees, or close to it, would mean the city  would lose about 775 acres of canopy cover — or more than 25% of what existed before the ash borer began its work. Colorado State University has put the estimate higher, at 98,000 trees in Boulder being ash.

Some ash trees will be removed even before they host the borer, as affected trees quickly dry out and become brittle, creating a safety hazard. A brittle branch can snap off and hit a Boulderite in the head.

Tree losses exceeding tree planting

The city, meanwhile, is trying to offset the tree loss.

“We plant about 500 trees a year throughout the city,” Ferguson said. “And we’re planting a diversity of species.”

Adding a diversity of species ensures that when the next pest arrives, it won’t affect such a large portion of the city’s urban canopy. Historically, neighborhoods were planted with the same tree throughout for aesthetic reasons. That practice has proved short-sighted.

Even with the plantings, however, the ash borer is working faster than city staff. According to a 2023 report on the state of Boulder’s urban canopy, “tree losses have exceeded tree planting every year since 2014 — the first full year after the Emerald Ash Borer was discovered in Boulder.”

This matters because trees are natural coolers. As part of a nationwide urban heat mapping project, the National Oceanic and Atmospheric Administration (NOAA) provided Boulder a grant last summer to determine which areas of the city are more prone to extreme heat. Led by Cool Boulder, a city initiative to combat climate change at a local level, the heat mapping revealed that areas of town with ample tree cover are significantly cooler than those with more paved surfaces. 

Some parts of town — like the 28th Street corridor where asphalt is prevalent and trees are few — were almost 20 degrees hotter than others in the summer. The mapping also showed that neighborhoods with few trees generate so much heat it spills into the surrounding neighborhoods, and areas of town with ample tree cover cooled adjoining neighborhoods.

Emerald Ash Borer larva boring through a trunk. Courtesy of the Pennsylvania Department of Conservation and Natural Resources

As Boulder gets hotter from climate change, the heat mapping findings suggest we need to plant more trees, and sooner than later, because it’s not saplings cooling things down. The older neighborhoods with older trees were cooler than others. Yet those older neighborhoods are also more expensive — another instance of climate change bringing equity to the forefront.

Tree tender training

Building on this finding, the PLAY Boulder Foundation, in partnership with the city’s Parks and Rec department, is hosting a “tree tender training” in August with the explicit goal of helping to increase the city’s tree canopy while educating residents about this mounting problem. The training, which costs $40 a person, will include education in “identification, biology, common myths, care and pruning, planting and irrigation,” and more. Registration is now open.

Ferguson said the city will also be partnering with PLAY Boulder for a tree sale that will begin in August, with trees arriving in September and October.

Tim Drugan is the climate and environment reporter for Boulder Reporting Lab, covering wildfires, water and other climate-related issues for Boulder with a focus on explanatory and solutions journalism. He also is the lead writer of BRL Today, our morning newsletter. Tim grew up in New Hampshire and graduated from UNH with a degree in English/Journalism. Email:

Join the Conversation


  1. So a few additional points about planting and tending more trees. A common barrier to lowering landscape water usage through removal of blue grass is the presence of mature thirsty trees in the yard. I hope the city is recommending replacement tree varieties that are not water hogs. Is there a list? And the more dense the housing the less space for trees and greenery. Not a co-incidence that dense apartment buildings with no shade and lots of black roadways are the hottest parts of town. It becomes a question of shoehorning more housing into existing neighborhoods versus having a green and more comfortable environment. Any guess about which is valued more at this point? And should our building code require enough tree cover to cool new buildings? With a reduction of housing units in favor of green space as a result? I think future Boulderites would really appreciate our foresight as summers consistently become 100 degree days.

  2. Unfortunately, urban planners and developers did not learn from the Dutch Elm problem from the 1970s. We were the first owner of a home in Westminster where almost every tree planted in our subdivision was an ash tree. City planners need to require developers to plant a variety of trees when approving new subdivisions.

Leave a comment
Boulder Reporting Lab comments policy
All comments require an editor's review. BRL reserves the right to delete or turn off comments at any time. Please read our comments policy before commenting.

Your email address will not be published. Required fields are marked *