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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.

The term native is increasingly being used in horticulture and tree planting practices, but what does it mean and how does it impact Denver’s urban forest? The term native refers to plants that have historically existed in an area and sometimes goes further to only include plants that existed prior to European colonization of North America. The term non-native means plants that have been introduced through human activities. Non-native plants can become naturalized through establishing and reproducing on their own in the new environment. These plants are typically not harmful to the surrounding ecosystem; however, some species can become invasive and cause environmental or economic harm through rapid spreading.

Native plants are important to local ecosystems because they are well-adapted to the soils and water availability of the area, support wildlife and pollinators and participate in symbiotic relationships and increased biodiversity. Denver is known for its semi-arid, high-plains climate and, prior to tree planting projects, was characterized by cottonwoods and willows growing near streams. Because Denver does not historically have many trees and the trees that are native are not well-suited for urban spaces, the way we plan for the urban forest today relies on non-native species.

Denver sees extreme temperature swings, periods of limited water availability and weather events such as hail and snowstorms, making it a challenging place to grow and establish trees. Through our free tree planting program, we have focused on selecting species that are hardy, add diversity to the urban environment and are resistant to local pest and disease issues. Over the years, the mindset of planting the same species in lines along streets and blocks has shifted to prevent major tree loss events such as the effects of emerald ash borer reaching Boulder in 2013.

To view Denver’s rules and regulations for tree planting within the City right-of-way, as well as the 2025 Approved Street Tree List, please visit our website at denvergov.org/forestry.

In 2024, we planted over 80 different species and cultivars of trees which will support a healthy urban forest for future years. While native trees and plants are important to maintain in our city, selecting a tree for your property or plants for your yard both rely on planting the right species in the right place. This selection process will vary based on your individual site characteristics but will continue to change with differing weather patterns and temperatures to come. Continued research and understanding of available species, as well as development of new cultivars, will help aid in decision making and creation of climate tolerant trees.

For Additional Resources, Visit the Links Below:

Rollinger Tree Collection

CSU Extension Native Trees for Colorado Landscapes

USU Extension Native Tree Discussion

You may be familiar with the program formerly referred to as “Ash GAP Removal and Replacement,” which was started in 2020 to address a “gap” in our response to the invasive Emerald Ash Borer (EAB). The program removes small ash trees in poor condition to reduce host material for EAB. This program, along with our Be a Smart Ash Planting Program and Be a Smart Ash Treatment Program, aims to lessen the damage caused by EAB when it eventually reaches Denver.

In 2024, we refreshed the GAP program name to Ash Removal & Replacement and updated the process of accepting applications and scheduling removals. The no-cost program targets small, poor condition ash trees in the public right-of-way within the City and County of Denver and accepts applications year-round.

To improve the efficiency and timeliness of assessing ash trees and scheduling removals, beginning in 2024, the program will alternate between west and east Denver annually. In 2024, we will operate the program in west Denver, and in 2025, we will focus on east Denver. Moving forward, removals in west Denver will occur during even years and removals in east Denver will occur during odd years.

EAB has been confirmed in multiple front range Colorado cities but has not yet been identified in Denver. We hope our proactive efforts in Denver reduce EAB’s impact on our urban forest. To learn more about the Ash Removal & Replacement program and eligibility requirements, please visit our page here: https://beasmartash.org/ashreplacement/. If you have questions about the program or a forestry-related issue, contact us at Tree@denvergov.org or 720.913.0651.

Between life, work and home, we’re all busy bees these days. It seems like there is always one more thing on our never-ending list of to-dos. But when it comes to your home garden, we want to share an idea from Denver’s Office of the City Forester that will help keep your scene serene. Because keeping things pristine? That takes too much time. So, give yourself a rest and leave a little mess!

Think about the pollinators.

Flowers, fruits and leaves are great for pollinators. Having plenty of flowers and flowering trees around for pollinators is crucial to healthy pollinator populations, which are necessary for food production that we need as humans. It’s all connected.

Flowers, fruit and leaves are also good food sources and habitats for our insects and wildlife. Plus, embracing the messier side of landscaping is vital to maintaining a healthy urban ecosystem, because it helps to balance diseases and pests.

Some trees attract bees.

In 2022 we planted this linden tree on request, as the property owner had a beehive and wanted them to have linden flowers to make their honey.

Leave some leaves.

Collecting leaves is a rite of fall. Raked piles make the cutest landing pads for our little ones and the colors somehow make the chore seem less like work and more like a nice way to get some cool crisp air in your lungs and not feel like you’re actually getting exercise too! But just say no to plastic and instead, responsibly dispose of leaves in compostable paper bags. You can get them for free from our LeafDrop program.

But don’t work too hard – try to leave a few leaves behind. Do a pass or two with your electric mower to chop up the leaves into small pieces for faster return of organic matter into the soil Creating compost from your organic food scraps and yard waste, like leaves, is a great way to make fuel for your trees, lawn and flower beds. Just add a little in when refreshing your mulch.

Besides leaves, how else do trees make an embraceable mess?

Some trees make their mess by dropping seeds, so they can adapt and reproduce the next generation. This post from our friends across the pond tells us about the five ways trees spread their seeds: the plant disperses them by gravity or force, or they spread seeds with a little help from animals, wind or water. Cleaning up after these trees hampers their hard work to survive.

As always, if you need more information, please contact Denver’s Office of the City Forester at forestry@denvergov.org or 720-913-0651.

Denver Parks & Recreation has worked with nearly 20,000 volunteers across recreation and urban & mountain parks, this year alone. By partnering with dozens of local and state-wide programs, DPR is fostering current and future generations of environmental stewards. If you have a passion for the outdoors, get involved building a more resilient future today.

Regardless of what the thermometer reads, it appears that summer is coming to an end –– which means that fall will soon bring us the gift of cooler breezes, colorful leaves, and the desire to engage in all the cozy things. It’s a beautiful season to get outside in Colorado, and the perfect time to plant a tree and contribute to our urban canopy.

Fall is a great time to plant new trees!

People tend to think of spring as the time to plant trees, but the truth is, fall can be even better! The heat of summer has left behind healthy soil, ready for planting. And there are plenty of sunny days between now and the first freeze to get those roots growing strong. There is always more to learn, but we’ve narrowed it down to some of the basics here: Everything You Need to Know About Planting a New Tree.

Where should I plant my tree?

Make sure there’s room for a new tree in your public right-of-way, especially if you’re applying for a free tree. We recommend following these spacing guidelines whether you’re applying for a free tree or planting on private property. Basic where-to-plant guidelines:

• 30 feet from the outside edge of intersecting curbs, to preserve the sight triangle
• 35 feet from large shade trees
• 25 feet from ornamental trees
• 20 feet from streetlights
• 20 feet from stop signs
• 10 feet from edge of alleys, driveways and fire hydrants
• 7 feet behind attached sidewalks
• 5 feet minimum from buried electric lines and plant ornamental trees under overhead power lines
• 5 feet minimum from water meters/pits, gas lines[MJPDCM2] , but 10’ preferred
• Centered within tree lawns/planters

How mu(l)ch is too mu(l)ch?

Applying mulch around the tree before it settles in for its winter’s nap is a very good idea. A mulch ring around your tree holds moisture in and protects against temperature extremes. It also decomposes and adds nutrients to the soil, too. We recommend learning the right way to mulch before getting started.

Fall Color 101

While the name Colorado technically means “colored red,” Denver’s variable fall weather makes it tough for that to ring true in our urban canopy. Sure, we have trees that could turn red in fall, but they don’t! The reason why? Red fall color relies on certain environmental conditions, such as a gradual change in temperature…and we all know those early snows in fall feel far from gradual. Plus, many of our favorite and recommended species tend towards yellow or brown fall colors, as these trees can handle the rapid temperature and weather changes we experience.

Tips for trees in cooler months:

Successful fall planting is a critical combination of good timing, a healthy tree and a simple care plan. Following these three tips, your trees will take root in no time.

1. Good Timing – The best time to plant your fall tree is generally through October, but at minimum when nighttime temperatures are consistently above freezing. Planting a dormant tree can ensure that all the tree’s energy goes to establishing, so your tree will be well-rooted by spring.
2. The Right Tree – From the Kentucky Coffee Tree to the Turkish Filbert, there are many trees that thrive in Denver’s climate, each lending its own beauty to our important urban forest. Tree diversity is the healthiest way to combat future pests and disease in our urban forest.
3. Quality Care – Trees are dormant in the winter, but they still need you to keep them watered because the roots continue to grow even with what you see above ground appears to be taking its long winter’s nap. Contrary to what we’ve been told, snowmelt isn’t enough. Trees need care and watering year-round, especially in Denver’s arid climate. Just how much water is determined by the tree’s diameter. The formula is simple: 10 gallons of water for every inch of diameter of tree, with the frequency changing seasonally. (So for a 2-inch diameter tree, you need to water with 20 gallons 1-2 times a month in the winter, and more often during periods of drought (yes, we can get droughts in the winter!) Also, don’t forget to mulch, and watch for sun scald, too!

So, if you plan to plant new trees, you can use the fall season to get them going. It’s easier than you think! But if you need additional help getting going, here are some resources to find a tree care professional in Colorado.

As always, if you need more information, please contact Denver’s Office of the City Forester at forestry@denvergov.org or 720-913-0651.

Dutch elm disease is a fungal disease vectored in part by the elm bark beetle or spread through overlapping roots among elm trees. The disease is known to be lethal to elm trees, and it can rapidly infect neighboring trees of the same species. This disease was first discovered in the U.S. nearly a century ago – brought to the U.S. in European wood that was imported for fine furniture-making. It continues to threaten our elm trees today. Dutch elm disease is essentially incurable, so the best course of action is a hefty combination of diligence and prevention.

A term that may be new to readers is vector which comes from the Latin, vehere – which means “to carry.” Think about it like this: a vector is a bug that carries disease to people and animals. For example, consider how mosquitos and ticks spread diseases that can harm humans and pets. A beetle vector is a beetle that transmits disease, but to plants and trees. Elm bark beetles are referred to as beetle vectors of elm trees. They feast under the elm tree bark, which then spreads fungus into the tree’s “veins,” making the tree very sick and rapid decline soon follows.

These infections are worse when they take root in early spring, when the wood is still moist. But it’s still destructive in the summer. Either way, without early preventative mitigation, the tree dies.

When the tree becomes infected, the leaves in the high branches – or crown – begin to yellow and wilt.  Quickly, the leaves turn brown, curl up and then the entire branch dies. Branch by branch, the disease spreads until the entire tree dies. The American Phytopathological Society (APS) shares this simple-to-understand illustration on the cycle of Dutch elm disease:

Sadly, if you’ve already identified the disease, it may be too late for the tree. But there are still some steps you can take to keep the fungus from spreading.

Like most tree ailments, it’s better to prevent the arrival of an illness than to cure a diseased tree. Prevention involves following good cultural practices, such as not planting too many of the same species of tree in a row, proper watering, avoiding damage to the tree, mulching, and rotational pruning. Property owners should not attempt to diagnose, prune or remove elms that may be infected, as that may unintentionally spread the disease due to beetle or fungus existing in the debris. It’s always best to have a licensed tree contractor perform tree work and remove all debris.

If Denver residents rally to prevent the spread and save the elms, we’ll see success. Below are a few photos of a local tree with Dutch elm disease at the crown. Look familiar? If you see one in a public right of way, please call 720-913-0651 or email forestry@denvergov.org.

In the spring of 2023, Denver Parks and Recreation and the Office of the City Forester conducted a comprehensive community-wide survey to gather insights from residents regarding their priorities concerning trees and the urban forest across town. This valuable information is currently being utilized to craft the city’s inaugural Urban Forest Strategic Plan. The plan encompasses all trees within the city, whether on public or private property, and aims to enhance existing goals, introduce new ones, and establish a shared and fair vision for the future of the urban forest.

Trees provide numerous benefits, such as purifying the air we breathe, mitigating the heat island effect, and reducing noise pollution. They offer a welcoming environment for outdoor activities, contributing to better physical and mental well-being for residents. Moreover, a thriving urban forest enhances property values, fosters a sense of community pride, and attracts visitors, boosting the local economy. By caring for and investing in our urban forest, residents can actively contribute to a greener, healthier, and more sustainable city for generations to come.

Building on the knowledge gained from the spring survey, we now have a few additional crucial questions to ask. Your input will continue to play a vital role in shaping the plan, and we highly value your participation. Complete survey #2 by September 4, 2023.

With more people relocating to Denver, warming weather and more frequent periods of drought – we are tapping into our most precious resource like never before. Currently, about 40 percent of water in Denver is used for outdoor landscapes but water isn’t an unlimited resource. That’s why Denver Parks & Recreation has a plan for water efficiency, reuse and response.