Denver City Forester Mike Swanson recently spoke with Rocky Mountain PBS on how the lack of trees in certain areas of Denver has resulted in the creation of heat islands. Heat islands are urbanized areas that experience higher temperatures than outlying areas.
How does Denver’s Office of the City Forester plan to combat the heat? By planting trees, of course.
One in six 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 (EAB).
EAB feasts only on ash trees (Genus Fraxinus), including the green and white varieties which are most common in the City and County of Denver.
How to identify an ash tree:
While this identification method may not be as helpful when it comes to young ash trees (they typically have smoother bark), the bark on more mature ash trees tends to have distinct diamond patterns.
A simple leaf is defined as a single leaf that has a bud at the base of the leaf stem. Conversely, a compound leaf (the sort you’re looking for on an ash tree) is defined as having more than one leaflet per leaf connecting to a stem that has a bud at its base. Ash tree leaves typically have 5-9 leaflets per leaf. This photo is showing one ash leaf, with 7 leaflets.
By opposite branching, we mean the branches protruding from tree limbs have a mate protruding from the exact opposite side of the same limb. Only ash, maple, dogwood and horse chestnut trees have opposite branching.
How to tell if an ash tree is infested with EAB:
There are several signs to be aware of. S-shaped tunnels can be seen on the trunk under the bark layer and tiny, D-shaped exit holes are often visible in the bark. Additionally, impacted trees show signs of thinning and bark shedding. Dying ash trees also attract Northern Flickers, a type of large, brown woodpecker. The tree takes time to show signs of stress and often dieback begins towards the top of the tree, so it can take 2-4 years for trees to show any signs of infestation.
What does the emerald ash borer look like:
It’s small enough to fit on a penny, so you are unlikely to get a chance to examine one individually. If you do, the adult EAB is identifiable by its dark, metallic-green color and coppery-red or purple abdomen, which can be seen under its wings.
What should you do if you have an ash tree or discover EAB? Go here for more information.
Una de las mejores maneras de combatir al barrenador esmeralda del fresno (Agrilus planipennis) y ampliar el valioso arbolado de Denver es plantar un árbol en su propiedad. Gracias a la Oficina de Silvicultura de Denver, los propietarios que viven en la ciudad tienen tres maneras de obtener un árbol gratis.
Solicite un árbol gratis
Desde el inicio de la campaña “Be a Smart Ash”, la Oficina de Silvicultura de Denver ha plantado más de 10,000 árboles gratis para los propietarios de la ciudad que tienen espacio suficiente en la extensión de vía pública que corresponde a su propiedad. Es fácil hacer la solicitud en línea. Un profesional certificado en arboricultura evaluará minuciosamente su espacio y determinará el mejor tipo de árbol para su propiedad. Si usted reúne los requisitos, le entregarán el nuevo árbol y lo plantarán. Además, le darán instrucciones específicas sobre cómo brindar los mejores cuidados inmediatos a su árbol para que crezca sano. El programa está abierto a propietarios (como empresas y comunidades de propietarios) de toda la ciudad de Denver.
Plan para los espacios vacíos
Dado que en Denver 1 de cada 6 árboles es un fresno, es indudable que cuando aparezca el barrenador esmeralda del fresno se perderán algunos ejemplares, con lo cual quedarán espacios vacíos notables en nuestro bello arbolado. Con la meta de reducir la multiplicación de estos inevitables espacios vacíos, la Oficina de Silvicultura de Denver puso en marcha el Programa de extracción y reemplazo de fresnos, para retirar y sustituir los ejemplares más pequeños y en malas condiciones que hay en la vía pública en toda la ciudad. Actualmente, el programa se está llevando a cabo en las zonas sudoeste y nordeste de Denver, pero cualquier persona que esté interesada puede presentar una solicitud en línea.
Iniciativa comunitaria por la silvicultura
En 2021, la Oficina de Silvicultura de Denver inauguró la Iniciativa comunitaria por la silvicultura, que se dedica a podar o extraer los árboles que representan un riesgo para la seguridad del público. La iniciativa también contempla la plantación de árboles en la vía pública, si lo permite el espacio. Actualmente, el programa se está llevando a cabo en determinados vecindarios del sudoeste de Denver. Los propietarios que reúnan los requisitos para obtener un árbol gratis o servicios de mantenimiento para un árbol plantado en su propiedad recibirán una carta y una postal de parte de la Oficina de Silvicultura de Denver, en la cual se detallará a qué servicio o servicios pueden acceder y cómo solicitarlos. Sin embargo, tenga en cuenta que todos los habitantes de Denver tienen derecho a la plantación de un árbol gratis. Por lo tanto, los propietarios pueden presentar una solicitud en línea si consideran que tienen espacio en la extensión de vía pública que les corresponde.
Since the launch of Be a Smart Ash, the Office of the City Forester has planted more than 10,000 free trees for Denver property owners who have space for a tree in their public right-of-way. Applying online is easy, and an arborist or tree professional will carefully evaluate your space, determine the best kind of tree for your property and, if you qualify, the new tree will be delivered and planted along with specific instructions for how to immediately provide the best care for your tree to ensure a healthy future. This program is open to property owners, including businesses and condominium associations, across the city of Denver.
Since 1 in 6 trees in Denver is an ash, it’s certain that when EAB is discovered here there will be some tree loss, which will leave noticeable gaps in our beautiful tree canopy. In an effort to proactively combat these inevitable gaps, the Office of the City Forester launched the Ash Tree GAP Removal & Replacement Program to remove and replace smaller, poor-condition ash trees in the public right-of-way throughout the city. Currently, the program is focused on Denver’s southwest and northeast neighborhoods, although anyone interested in this program can apply online.
March 2022 Update: We are experiencing delays with GAP Program removals. Thanks for your patience.
Forestry Neighborhood Initiative
In 2021, Denver’s Office of the City Forester launched the Denver Forestry Neighborhood Initiative, which is dedicated to pruning or removing trees that pose a risk to public safety. This initiative also includes planting trees in the public right-of-way, as space allows. The program is currently focused on specific neighborhoods in southwest Denver. Property owners who qualify for a free tree or for tree maintenance at their property will receive a letter and a postcard from the Office of the City Forester outlining which service(s) they qualify for and how to claim them. However, everyone in Denver is eligible for a free tree planting, so property owners can apply online if they think they have space in their public right-of-way.
It’s believed that EAB likely arrived in the United States on lumber in cargo ships originating from its native Asia. It has killed hundreds of millions of ash trees in North America and has been discovered in over 35 states, including Colorado. In fact, it’s been found in surrounding metro areas including Boulder and Arvada, so it’s just a matter of time before it is discovered in Denver. All ash trees that are infested with EAB eventually die because the larvae feed under the bark of ash trees, disrupting the tree’s ability to transport water and nutrients. Because of this, the dieback of the tree is a slow process, starting at the top of the tree. Since one in six of Denver’s trees are ash, Denver’s Office of the City Forester has been proactively preparing and planning for the EAB’s inevitable arrival for more than five years through planting, tree removal and replacement, and treatment activities.
Be A Smart Ash Program
With guidance from management practices such as “Slow Ash Mortality” and experiences learned from states like Michigan that have battled EAB for nearly 15 years, the Office of the City Forester created the Be A Smart Ash program in 2016 to drive awareness and inspire the community to take action to protect the ash trees on their property. Depending on the condition of the ash tree, homeowners are encouraged to either treat their tree with insecticide or remove and replace it with a new, non-ash tree. The arborists at the Office of the City Forester are happy to assist homeowners in determining the best care for their trees.
The only way to ensure that ash trees survive EAB is to have them treated with insecticide, of which there are a variety of options. Homeowners can visit BeASmartAsh.org to find out more information on how to treat their private property trees. For public ash trees, the Office of the City Forester is actively taking steps to treat ash trees in public spaces and in the public right-of-way within our city. The Forestry team is now on Cycle Six of treatment, meaning many trees in public spaces or in the public right-of-way are receiving repeated treatment.
If homeowners decide to keep and maintain their ash trees, there are three main treatment methods: trunk injection, soil drench or bark spray.
Trunk Injection: A licensed pesticide applicator drills a hole in the base of the tree and injects the tree with an insecticide. The insecticide is conducted by the tree’s vascular system and ultimately into the cambium layer. Since the EAB larvae hatch and feed on the cambium layer, this stops the spread by preventing the larvae from becoming reproductive adults. When properly administered by a licensed tree care professional, some trunk injections have shown to be 90% effective at controlling EAB, making this the most effective treatment option.
Soil Drench: A licensed pesticide applicator will clear the area beneath a tree and then apply liquid or granules that are then watered-in. This allows the tree to soak up insecticide through its root system.
Bark Spray: A licensed pesticide applicator sprays up to 18 inches from the soil up the trunk. This insecticide soaks through the bark and is transferred to the leaves of the tree. This helps to stop the spread of EAB as it targets the adults who feed on the leaves of the tree.
Removal and Replacement
Ash trees that are left untreated will eventually succumb to EAB. With this in mind, it may be more effective to remove ash trees that are already dead or dying and replace them with new, healthy trees (not another ash). In 2020, Denver’s Office of the City Forester created the Ash GAP Program to remove and replace smaller, poor-condition ash trees in the public right-of-way throughout the city, encouraging homeowners in target neighborhoods to opt-in to the program. The City is targeting small diameter ash trees in poor condition because trees that are already stressed are particularly vulnerable to EAB. It is also recommended to remove smaller ash trees and replace them with a species not susceptible to EAB, as ashes will need proactive EAB treatment throughout their entire life in order to survive an EAB infestation.
In 2021, the program is focusing on Denver’s southwest and northeast neighborhoods, although anyone interested in this program can apply at beasmartash.org/ashreplacement. Removing unhealthy ash trees from the urban forest not only promotes a healthy forest, it removes a food source that EAB would feed on while eliminating unhealthy trees that would be targets for EAB.
Of course, planting and maintaining new trees is the best way to have a resilient, healthy forest. New plantings will help fill any gaps in the canopy after the demise of ash trees from EAB. Planting for diversity also increases the resistance of Denver’s urban forest to any potential future pests. The Office of the City Forester has always encouraged qualifying homeowners to apply for a free tree. Since the start of the BASA program along with other programs, the Forestry team has planted over 2,000 trees a year in Denver. Any Denver resident who has adequate space in their public right-of-way is encouraged to apply for a free tree at beasmartash.org/freetree.
Want to learn more? Become a Smart Ash and join us in our fight against EAB!
By Urban Forestry Operations Assistant Paul Cancik
With spring upon us, many residents are eagerly planning their gardens and starting to spruce up their yards. Pruning helps trees live longer, which allows them to grow taller and contribute to Denver’s urban canopy. With this in mind, Denver’s Office of the City Forester is offering helpful tips for pruning. It’s important to keep in mind that if you cannot safely prune your tree from the ground, it’s best to hire a licensed tree care professional since they use specialized equipment and have the necessary field knowledge. When you prune a tree, you are planning for the future, and with patience, you will ultimately have results that benefit generations to come.
Why should you prune your trees?
Pruning helps ensure that your tree develops a strong form/structure and prevents breakage in the future.
Thinning your tree makes the crown (top) healthier by allowing more air and sunlight to pass through it.
Pruning, much like watering, helps give your tree longevity; future generations will be able to enjoy it.
Removing deadwood from your tree helps prevent insect infestation.
If pruning is neglected, a tree can become susceptible to breakage, making the tree potentially dangerous.
What should you prune from your trees?
Follow the “3 D’s” of pruning: only remove Dead, Damaged and Diseased wood, especially if the tree is not established. You can also prune branches that impact the structural integrity of the tree.
Be deliberate about what you prune from a tree.
It’s important to prune around stop signs and to ensure sidewalks are clear to prevent accidents on or near your property. Stop signs should be clearly visible and sidewalks free of obstructions. The clearance requirements in Denver are 8’ above sidewalks and 13.5’ above streets and alleys.
When should you prune your trees?
While you may prune your tree year-round, ideally the best time to prune is late in the dormant season or early spring, before leaves form. This is typically a good time to remove excess or undesired branches because the tree is not putting forth energy to create foliage.
Certain trees, including American elm (Dutch elm disease) and fruit trees in the rose family (fire blight) should only be pruned while dormant to reduce the spread of disease.
Only prune a young tree two years after it has been planted and just focus on dead, broken, crossing and interfering branches.
Tips for pruning:
Make sure that every pruning cut you make is clean and smooth. The best tool to use for pruning is a pair of sharp bypass hand pruners for one-inch branches because they make smaller cuts that the tree can recover from faster.
Colorado has a very short growing season compared to other regions. A shorter growing season means the tree has a shorter period of time to create and store energy, which ultimately affects how quickly a tree can recover from pruning. A young established tree can tolerate removal of 1/3 of its foliage in a growing season. A mature tree should never have more than 25% of its live foliage removed in one growing season.
If you are pruning something off your tree that you can’t reach from the ground, it’s advised that you hire a tree care professional since they use specialized equipment such as an aerial lift truck/bucket truck and they have the needed field knowledge and expertise. In the City of Denver, tree contractors are required to be licensed and insured. A list of Denver’s licensed tree contractors can be found by visiting https://www.denvergov.org/forestry.
If you suspect an insect problem, contact a tree care professional to develop the most effective and environmentally conscious solution.
Covering a wound or using wound dressings is not recommended and may be detrimental to tree health.
By Michael Swanson, City Forester, Denver Parks and Recreation
As winter approaches, many Denverites are preparing for the cold months ahead by blowing out their sprinkler systems and composting their leaves. But many mistakenly think their trees are self-sufficient and that the snow, when it comes, will provide enough moisture to sustain them until spring. That’s unfortunately not the case. Trees need water year-round, especially in our arid climate.
Here are a few signs that your trees may be in distress and tips for how to winterize your trees to help them stay healthy year-round:
Signs of tree in distress:
Trees that exhibit premature autumn color or shed leaves prematurely
Advantageous shoot growth along the branches and/or trunk of the tree or near the base of tree
Deadwood throughout the crown of the tree
Tips for your trees in winter months:
Use 20 gallons of water per week for every inch of diameter of tree (for instance, a 2-inch diameter tree needs 40 gallons of water in a one-week period). A new tree will need focused watering atop the root ball and then throughout the rest of the planting area.
Don’t forget that even when it’s 40 degrees or warmer, your tree still requires watering.
Apply mulch around the tree, leaving a six-inch gap between the tree trunk and the start of the mulch; mulch should extend by a three-to-four-foot radius from the trunk of the tree or to dripline (whichever comes first).
Portions of Southern or Southwestern-facing trees are subject to sun scald. Sun scald is the damage that occurs to living cells just underneath the bark of a tree due to the day-and-night fluctuations during Colorado’s winter months. Signs of damage include discolored and/or cracked bark or sunken areas within the bark. This is a serious byproduct of our warm winter days. To avoid this, you can wrap your tree’s trunk using materials such as cloth or tree wrap available at a hardware store. Butcher paper is a great tool as it’s waterproof and removes/absorbs some of the energy that the sun produces.
Keep an eye out for signs of emerald ash borer (EAB), which has destroyed millions of ash trees in the Midwest, has been discovered in Boulder and recently in Arvada and will inevitably arrive in Denver sometime in the near future. There are an estimated 1.45 million ash trees in the Denver metro area, including 330,000 in the City and County of Denver. That means that one in six Denver trees are ash trees, and they can be found everywhere in the Mile High City – with the majority on residential properties.
Become familiar with the Be A Smart Ash campaign, which aims to actively educate and enlist the help of you – our City and County of Denver residents – in the process of identifying, treating and replacing ash trees.
Water device (such as a soft spray wand) for winter watering (can also be used year-round)
Soil moisture meter (it tells you when the soil is dry and needs water)
A hose (make sure it’s the right length to reach your trees if they are a long distance from your spigot)
Pruning shears to remove dead wood
Compost comprised of organic food scraps and yard waste (use it on your trees, grass or flower beds when you are refreshing your mulch; when using compost with your trees, do not apply more than a quarter inch per year within the dripline of the tree, and make sure the compost does not come into contact with the trunk of the tree or roots)
William Carlos Williams once eloquently wrote: “…having prepared their buds against a sure winter, the wise trees stand sleeping in the cold.” Let’s help our trees prepare for winter slumber so we can enjoy their healthy urban canopy next year and for years to come.
Grab your family and friends for this interactive, educational show from the Be A Smart Ash team. Each livestream brings the natural world of Denver into the comfort and safety of your living room. The third episode in the series debuted on Friday, October 16 at 11 AM on the Denver Parks & Recreation Facebook page, but you can watch the episode in its entirety above.
In Episode 3, leaf-peeping season is in full swing, and we want to help you enjoy it. We discuss the importance of winter watering for your trees, explore the beautiful fall colors and provide tips about how to protect your ash trees from Emerald Ash Borer. We also explain how you can turn your fall leaves into EcoGro compost, available locally from Ace Hardware. See previous episodes here.
Lexi Brewer, Urban Forestry Operations Assistant, City and County of Denver
As an Operations Assistant of Denver’s Office of the City Forester and a member of the Be a Smart Ash (BASA) team, I have loved being a part of a program that gives away free trees to Denver residents. But a lot of behind-the-scenes work happens to make our streets and neighborhoods green and shaded. In this blog post, I’ll describe our program and how you can help us grow Denver’s urban forest.
Why Plant Trees?
Tree planting is just one part of our BASA program. BASA started in 2016 as a holistic way to stop the spread of Emerald Ash Borer (EAB) by making our urban forest as resilient as possible. The EAB is a non-native insect that targets Ash trees (Fraxinus genus). The larvae of EAB burrow under the bark of the tree, which eventually prevents the tree from being able to transport the necessary nutrients between its leaves and roots for survival, ultimately killing the tree. In Denver, this pest will have devastating consequences, as approximately one in six of all of our trees are ash trees. To reduce and slow the impacts of EAB, our BASA program 1) treats existing ash trees with pesticide, hoping to increase their resistance to EAB, and 2) plants new, diverse trees that will help to fill the holes in the canopy after the death of any ash trees. Planting for diversity will also increase the resistance of our canopy to any potential future pests. Here’s how our planting program works:
Step 1: Spread the word!
Our BASA program is an “opt-in” program, which means that homeowners have to hear about the program and apply either online or with a mail-in postcard. We rely on neighbors and community partners to spread the word about our program and increase the number of people who apply – which will increase the total number of trees that we’re able to plant!
Step 2: Choose where to plant
First, we have to understand which areas of the city could benefit the most from our free tree service. To do this we work with community partners and researchers to create and analyze the data that we need. We prioritize our target planting areas based on lowest canopy cover and lowest annual household income per neighborhood, trying to service first those who could benefit the most from free trees.
this video, one of our partners, Sylvia Leon Guerrero, discusses the
benefits of trees, and how her research used GIS mapping to locate the areas of
the city where trees could increase walkability and have other tremendous
To determine exactly where we should plant the tree(s) on each property, a member of our staff goes out to the property of the homeowner who made the request. We try to plant as many trees as possible onto a site, being sure to comply with the siting requirements of the city (see below). We also look around and make a species recommendation based on the surrounding trees, trying to increase the species diversity on each street as much as possible. We’ll also note the site conditions in order to recommend a tree that will have the highest chance of survival in the area. We always email the homeowner these recommendations and work with them to provide a species that they like.
Step 3: Plant the tree & ongoing care
After the tree placement and species have been approved by the homeowner, we then use contractors to drop off and plant the trees. From there, our work is done; however, the on-going care of the tree is critical to its survival. We depend on the homeowners to water and care for their tree. Proper watering includes watering with a hose two to three times per week in the summer, at least ten gallons each time (15 to 20 minutes) and two times per week throughout the winter when the soil is dry. We’re happy to answer any care questions that anyone has at email@example.com. With proper care, each tree brings us one step closer to a more beautiful, resilient, urban forest!
Knowing you Denverites are likely spending more time at home and in your yards than in previous summers, we want to help you learn more about how you can help protect and preserve Denver’s urban canopy and specifically, what the City Forestry team is doing this year to help combat a relentless ash tree insect.
You may be familiar with the Be A Smart Ash program, and you may have heard that the threat of Emerald Ash Borer (EAB) on 1 in 6 of Denver’s trees is imminent. The Q&A below will help explain what the City Forestry team is doing in select neighborhoods to combat EAB; i.e. removing ash trees in poor condition in your right-of-way and how they will replace that tree with a free tree – regardless of whether you have an ash tree – to help bolster and diversify our precious urban canopy.