Monthly Archives: June 2025

In mid-June, the City of Denver confirmed the presence of the emerald ash borer (EAB) — a wood-boring insect that poses a serious threat to our beloved ash trees. The EAB has already caused widespread devastation in 37 U.S. states and parts of Canada, and now it’s here. But with your help, we can protect Denver’s urban forest.

Why It Matters

Ash trees make up a big part of our green landscape. In fact, 1 in every 6 trees in Denver is an ash tree, and 1 in 10 trees on public land is an ash. These trees shade our neighborhoods, clean our air and enhance our parks. EAB attacks and kills ash trees, often within just a few years. If we don’t take action, Denver could lose hundreds of thousands of trees — and the shade, beauty and benefits they bring.

Denver launched the Be A Smart Ash campaign ten years ago to empower residents to protect their ash trees. Since then:

• 6,000 ash trees have been treated in parks and parkways
• More than 1,000 ash trees have been removed where needed
• About 1,500 new trees are planted every year, with more planned to grow our urban canopy

What You Can Do

Now we need you to take action! If you have an ash tree on your property or in the public right-of-way near your property, here is what you need to know and the steps to take:

1. Identify Your Trees: Early detection gives us the best chance to save a tree. To identify if you have ash trees on your property, look for compound leaves with five to nine leaflets, opposite branching and diamond-patterned bark. Check out photos and tips for identifying ash trees.
2. Inspect your Trees: Know the signs of EAB – look for D-shaped exit holes, falling bark, wavy lines under the bark or woodpecker activity (especially Northern Flickers).
3. Make an Action Plan: If you have an ash tree, find a licensed tree care professional to discuss your options for treatment or removal and replacement. Proper treatments are 95% effective when applied by professionals. If removal is necessary, consider replacing your ash tree with a Denver Office of the City Forester-approved species.
4. Spread the Word: Tell your friends and neighbors about EAB and what they should look for to help stop the spread of EAB.
5. Final Tip: Transporting firewood spreads EAB. Don’t do it! Keep wood local, and work with a licensed tree professional for any tree removal.

Trees are vital to our environment and quality of life, as well as our property values. While Denver has prepared for the arrival of EAB, we also need your help to slow its spread. Together, we can take action to preserve and grow our urban forest for generations to come.

Are you curious if your tree is growing properly and what you should expect to see in the coming years? Trees can vary in the time they take to grow and reach mature size. When a tree is first planted, it focuses its energy on acclimating to the new environment and developing its root system; we call this phase of a tree’s life “establishment.” Each planting site is unique in the type of soil, water availability, light and microclimate, requiring each tree to adapt and grow differently. While the tree is establishing and focusing on root growth, there will be very little, if any, growth in the trunk or tree canopy. During the establishment phase, trees are very susceptible to any type of stress and are at the highest risk for mortality. Following care guidelines, including routine watering, and adding mulch to the base of the tree will limit transplant shock and stress trees can experience after planting.

Newly planted trees typically take 1-2 years per inch in caliper before the tree is established, meaning its root system can now support the growth of the rest of the tree. For our Be A Smart Ash free tree planting program, we plant 1 ½” to 2” caliper trees that will typically take 2-3 years to establish. Only once established will trees begin to grow in canopy size and trunk diameter.

It is important to keep an eye on your tree to look for changes in growth or potential issues related to stress, pests, or disease. In ideal conditions, trees can grow 1-2 ft. in height and 1 in. in diameter annually. Denver does not always have “good” conditions, with poor soils, extreme temperature changes and limited precipitation that can all impact tree establishment and growth.  Although it can seem like your tree is slow growing, it is putting immense energy both above and below ground as it absorbs nutrients, transfers water, sequesters carbon and releases oxygen…all while adapting to Denver’s tough climate! By continuing to care for your tree, it will continue to grow and care for you by providing shade and environmental benefits for years to come.

Google image, BASA Catalpa tree in 2021 and again in 2024.