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Branching Out

Our Ash trees are under attack

There is an exotic beetle responsible for the death, and subsequent removal, of many trees across Boulder. The Emerald Ash Borer (EAB) larvae (the immature stage) feed on the inner bark of ash trees, disrupting the tree’s ability to transport water and nutrients. The dying trees quickly dry out and become brittle.

We love trees too, but safety is our priority

Trees must be removed before they become a safety hazard to people or infrastructure. For example, along the Boulder Creek Path you will notice tree removals near the path and the creek to prevent trees from declining to the point where they could fall on a path or in the creek causing possible harm or flooding.

Why now?

Staff have been monitoring the infestation of EAB and systematically have been removing and treating ash trees across Boulder. They have been trying to reduce the impact to the community; however, impacts of the EAB infestation will grow exponentially over the next few years.

Researchers have found almost 100 percent tree mortality from EAB in most species of North American ash especially green ash.

As ash trees die from EAB infestation and become brittle, the removal becomes increasingly expensive due to the precautions arborists need to take. For example, an arborist may no longer be able to climb the tree and must rely on vehicle with a bucket lift or crane to safely perform the job. It not only becomes more expensive but requires more space, which can be problematic for private homeowners with small backyards containing untreated ash trees. 

Current Project

Scope of Work

EAB has created the need for removals along the Boulder Creek Path Corridor. Boulder Forestry is removing the dead and dying trees infested with EAB. When stump grinding is not feasible, the outer edge of stumps will be hand-treated with an herbicide, which is necessary to prevent ash regrowth and to allow restoration efforts to establish. The herbicide contains a blue dye. A blue ring on the outer edge of the stump shows where the limited-use herbicide was applied. See 'Stump Treatment' below for more details.


Other removals of infested trees along the Boulder Creek corridor are anticipated. Staff will manage these areas to mitigate growth of invasive species while a full restoration plan is developed for replanting.

March 25-29, 2019
Forestry staff will remove 140 ash trees along the Boulder Creek Path along Boulder Creek between 9th and 6th Streets near Evert Pierson Kids’ Fishing Pond. All are ash treese affected by EAB; 80 trees less than 7 inches in diameter. 

The work will require a closure of the Boulder Creek Path in this area; the path will be closed 8 a.m. to 5 p.m. each day. Pedestrians and cyclists using the path will be detoured north of the creek to the path along Canyon Boulevard around the work zone. Signage will be on site to assist creek path users and to guide pedestrians and cyclists. Numerous parking spaces in the Boulder County parking lot at 6th Street and Canyon Boulevard will also be unavailable through-out the week due to the project. 

Nov. 14-16, 2018
Forestry staff will remove 59 ash and non-ash trees along the Boulder Creek Path adjacent to Scott Carpenter Park, from the southwest corner of the park upstream to 28th Street. Forty-two trees are ash affected by EAB; only 24 trees are larger than 7 inches in diameter. 

June 18 - 28, 2018
Forestry staff will remove 172 ash trees in the area between the Kid's Fishing Pond and 30th Street; the majority of the trees are under 7 inches in diameter, and only three trees are larger than 16 inches in diameter. 

*Please Note:  Dates are tentative. 
All park construction and restoration project dates are subject to change due to weather conditions, contractor schedules and material delivery. Updates and progress reports will be posted on the Parks and Recreation website as available.

Boulder Creek Path Detours

The work will require intermittent detours and closures on the Boulder Creek Path. Signage and/or flaggers will be on site to ensure public safety and to guide pedestrians and cyclists through work zones. 

Stump Treatment

As ash trees sprout back profusely when removed, impairing restoration efforts, stump removal is necessary to prevent new growth, to avoid having to return to the site to remove suckers and to allow new native plant material from restoration efforts to establish long-term. Forestry staff will attempt to grind tree stumps mechanically when appropriate. When stump grinding isn’t feasible, the stump will be hand-treated with an herbicide necessary to prevent regrowth. Because all pesticide use, including herbicides, is restricted along the Boulder Creek corridor within city limits, staff requested and was granted a city manager’s exemption that applies only to ash tree removals and is restricted to one product, imazapyr (Habitat®) that is the least toxic and most effective option. When treating a stump, staff will apply by hand to a small area around the very edge of the stump, right below the bark. A blue dye is added to the herbicide for more precise application to minimize non-target exposures. Staff will post signage in the work areas when this stump treatment is required.

EAB Prevention Treatment Options

EAB prevention treatment of private ash trees may still be an option. Please check with a  licensed arborist pdf for assessment of private trees.

If you have treated an ash tree in the street ROW, please notify staff. There is a process to notify staff to prevent the removal of a tree that has been treated.

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Environmental Services

Often times our trees recede into the background of every day and we forget how they help define the spaces we enjoy, keep us and our environment healthy, and even tempt us to climb into their branches. Do you know of all the amazing benefits they provide us as a community?  

Boulder's trees by the numbers


This project is part of a 2018 focus on ecosystems , the connections of living beings and environments, in Boulder and beyond, that can help us reduce GHG emissions, remove CO2 out of the atmosphere, and be more resilient to the effects of climate change. Trees, for example, capture atmospheric carbon while also providing temperature buffering and moisture retention.

While they do offer this ecosystem benefits that must be restored later in the project, ash trees are not native to this area. A comprehensive treatment approach provides an opportunity to restore, and even grow, this aspect of the Boulder ecosystem with native trees.

Community Partner

PLAY Boulder Foundation is an independent nonprofit working closely with the City to provide support. PLAY Boulder Foundation creates excellence in parks and recreation in Boulder, Colorado by mobilizing community support through education, philanthropy and advocacy. 

The Tree Trust was created by the PLAY Boulder Foundation in 2015 upon anticipation of the public-private partnership goals outlined in the City of Boulder Parks and Recreation’s Urban Forest Strategic Plan (UFSP).