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Winterize Your Trees

The new year is in full swing and as the winter weather ebbs and flows, Denver will continue to see freezing temperatures, flurries of precipitation and snowstorms potentially as often as we see sunny, 60-degree days. We caught up with the smart folks at Denver Parks and Recreation’s forestry office to discuss the best methods for winterizing the trees in your yard. For insight and expertise on how to protect your trees from Denver’s highly variable winter months, read on.

How should a homeowner protect a tree during the winter months?
Place mulch around the base of the tree, about three to four inches from dripline to dripline, if not three to four inches. Leave a three-inch gap from the edge of the mulch to the stem of the tree. Because we have warm winter days in Denver, too, watering your tree in the winter is also essential. Flood irrigation around the tree when daytime temperatures reach 45 degrees or higher (if the drip line lacks residual snow cover). In Colorado, winter watering months are typically November to February. Regardless of the age of the tree, this advice applies across the board.

Is there an ideal timeframe that Colorado homeowners should have their trees pruned each year?
Pruning can be done at any time. The dormant season is best for pruning, especially for fruit trees, but homeowners who do their own pruning should wait until they can determine the difference between live and dead material. Always hire a professional if you’re unsure. However, American elms and any tree in the rose family (crabapples, pears, plums, cherry, apple) should be pruned during the winter months. Fruit trees should also be pruned in the winter months to avoid the spread of fire blight. Pruning maple and walnut trees during winter months is not ideal because they will bleed sap from the pruning cuts.

Should people expect a certain level of damage to their trees during the winter or can it be entirely prevented?
Winter damage can be avoided if trees are regularly watered and properly mulched. Damage can occur in what we describe as “heavy load events” – a lot of snow breaking limbs – or a quick hard freeze, especially common for conifer trees. We saw this happen recently in October 2019 because of the two freezing nights.

When discussing winter damage, the term “dieback” comes up fairly often. What is dieback?
Dieback is a condition in a plant in which branches or shoots die from the tip inward; caused by many conditions, but particularly a drought condition if they are not properly watered.

If branches break in a snowstorm, what’s the best use of those that have fallen or broken?
We advise mulching them and re-using that material in your landscaping, flower beds and even around the base of your trees by chipping material using a brush chipper. Alternatively, Denver’s Department of Transportation & Infrastructure (formerly Denver Public Works) collects a limited amount of branches. These must be no larger than four inches in diameter, and they must be cut into lengths of four feet or less, bundled and tied, and weigh no more than 50 pounds. Customers may set out up to 10 bundles of branches on their scheduled every-four-week extra trash collection. Denver residents can also drop off branches at the Cherry Creek Recycling Drop-off center.

Do you have any other tree winterizing tips?
For young trees wrap the trunks each year with tree wrap, until the bark develops furrows. This is especially important with thin-barked species such as honey locust, maple, linden and fruit trees. Starting at the base, wrap the trunk covering each turn about 30% up to the first branch union of the canopy to protect them from the intense sunlight.

Keep Leaves Out of Landfills

Once again, we’re excited to partner with our friends at Denver Public Works, Denver Recycles and Ace Hardware to give away FREE 5-packs of compostable 30-gallon paper leaf bags to help Denver residents keep leaves out of landfills!

Unlike plastic bags, these brown paper bags can be composted along with the leaves; saving time and reducing plastic waste in our landfills. This fall, Be a Smart Ash and compost your leaves.

  1. Download a coupon for free and compost-friendly leaf bags.
  2. Redeem your leaf bag coupon at an Ace Hardware store near you by printing it or showing it to a cashier. See a map of participating locations here.
  3. Fill the paper bags with your leaves.
  4. Drop off your paper leaf bags at your nearest Denver LeafDrop site.

Attack of the Zombie Trees

Halloween is upon us! Do you know what’s spooky? Zombie trees.

Zombie trees are those that are still standing upright but are dead inside from a variety of environmental factors; disease, insect infestation, improper pruning and lack of water. They might look alive but these ticking time bombs will eventually topple over, posing a real danger to both the public and your property. If you have a zombie tree, a strong wind could easily send it toppling over and cost you thousands of dollars in damages – to your yard, roof, vehicles or wherever it may land.

Symptoms of a zombie tree:

  • Bark that is decaying, missing or showing breakage
  • Loss of leaves
  • Dead and/or falling branches

“Because we have early fall snows and high winds in Colorado, Denver residents should always be mindful of the health of the trees on their property,” said Sara Davis, Urban Forestry Program Manager in the Office of the City Forester.

If you live in Denver and own property, you are responsible for:

  • Cleanup of debris from trees on private property and from trees within the public right-of-way adjacent to their property. (Limbs on the ground are considered debris. Property owners can hire any company to haul limbs away – for this type of work, the company does NOT have to be a licensed tree contractor.)
  • Pruning needs of private property trees and trees within the public right-of-way adjacent to your property.

Unfortunately, there are times when the safety of the public necessitates that the work be completed immediately and Denver Forestry cannot allow time for the property owner to find a licensed tree professional. When a tree or limb is blocking safe access to the street or right-of-way, Denver Forestry has an on-call contractor remove the limb or tree and bills the property owner for the work.

There are many tree professionals that are licensed by the Office of the City Forester who can safely remove a zombie tree from your yard, sparing you the costly expense of a fallen tree. Before you hire someone to remove your zombie tree, ask if he or she has a Colorado State Department of Agriculture tree service license.

Take Advantage of the Fall Color Change

As pumpkin spice lattes make their debut at our local coffee shops, so do the colors of fall. The leaves of trees in our parks and backyards are beginning to change from green to yellow, purple and plum – a clear sign that winter will soon be here. 

Ash leaves

Take advantage of these vibrant signs and Be A Smart Ash.  
With the Emerald Ash Borer confirmed in the City of Westminster, directly neighboring the City of Denver to the west, now’s the time to Be A Smart Ash. Use the vibrant fall color change as your guide and get outside; look for the most colorful leaves, then use these telltale identifiers to confirm you’ve found an Ash: 

  • Compound leaves: 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) 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. 
  • Opposite branching: 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. 
  • Diamond-pattern bark: 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 advanced ash trees tends to have distinct diamond patterns. 

Once you’ve confirmed the tree is an Ash tree, make sure you don’t see any pesky green bugs around, or any wavy trail lines or d-shaped holes in the trunk. Multiple visuals can be found here. Hopefully, the tree is healthy, in which case there are multiple options to help keep it that way. 

If the tree is on public property, look for a tag that says “This Ash Is Covered” to know it’s been treated by the City Forestry Department. More information on the city’s treatment and replacement plan can be found at If the tree is on your personal property, check out your next steps here. 

Think you might have an EAB infestation? Take a lot of pictures and contact the City Forester at or 720.913.0651. 

Find the Smart Ash Superhero Around Town

Fighting Emerald Ash Borer

The Smart Ash Superhero is making the rounds to local libraries, recreation centers and public spaces to introduce more Denverites to the Emerald Ash Borer (EAB) and ways to Be A Smart Ash. Be on the lookout for the Smart Ash display and let us know where you see him by tagging @BeASmartAsh or using the hashtags #EAB and #BeASmartAsh on Twitter.

Join our effort to save Denver’s ash trees by talking to your neighbors, friends and co-workers about EAB and encourage people to visit for more information.

Emerald Ash Borer Detected in Broomfield

Photo courtesy of Colorado State Forest Service

The Emerald Ash Borer (EAB) was confirmed in the City and County of Broomfield, northwest of Denver, on Aug. 20, 2019 – the first confirmation of EAB in Colorado outside of the federal quarantine that primarily encompasses Boulder County.

A Broomfield resident familiar with the appearance of EAB due to Broomfield’s proactive public education campaign noticed a suspicious insect on their personal property near 136th Avenue and Main Street and contacted the Broomfield city forester. It is not known whether the pest arrived by natural spread or human transport such as firewood.

At least 15% of all urban and community trees in Colorado are ash species susceptible to being killed by EAB. EAB attacks and kills both stressed and healthy ash trees and is so aggressive that trees typically die within two to four years after infestation. While EAB has not yet been detected in Denver, it could already be here.

Wondering what you can do to protect your ash? We have resources to help you review options.

Sustainable Urbanism and Tree Care

Professor Austin Troy, photo courtesy of University of Colorado Denver

Austin Troy is Professor and Chair of the Department of Planning and Design at the University of Colorado Denver. As part of the Master of Urban and Regional Planning (MURP) faculty, Troy is one of the most respected researchers in the planning field. The Smart Ash team recently caught up with him to discuss sustainable urbanism and tree and lawn care during the hot summer months.

Question: How would you explain sustainable urbanism and regional sustainability to the average Denverite?

Answer: Urban sustainability is a field of study that examines the interaction of social and environmental systems in cities. I mainly work with green infrastructure – including mapping the location of trees and vegetation, prioritizing where they should be planted, and characterizing their many benefits to the city. As for regional sustainability, I like to point to the Green Building ordinance as an example. City voters passed this initiative in 2017 and it’s the most aggressive greening ordinance in the country right now.

Question: What’s your opinion of Denver’s current tree canopy?

Answer: It’s very impressive and vast when you consider how much the city has grown in the past century. Much of the current canopy can be attributed to decisions that were made 70 to 100 years ago by Mayor Speer and other Denver leaders.

Question: For homeowners, the million-dollar question is (water time slots and allocations notwithstanding) how much should you water your lawn? How do trees affect water needs?

Answer: Nobody quite understands this and the short answer is that it’s complicated. Tree shade is really important to overall irrigation demands. My team is in a partnership with Denver Water and when we look at their data we see that people who have mature tree canopy over grass can theoretically use less water in many cases than those with only grass. Broadly speaking, shade can offset your water costs. This may be because lawns without tree shade can get desiccated in the direct sun and homeowners try to compensate by overwatering. That said, we also think older trees are tapping into some of Denver’s older clay pipes for water. It’s a leaky infrastructure. Also, as people water their lawns, much of that water goes back into groundwater where more mature trees with deeper roots can reach it. There are a lot of factors to consider.

Question: What type of treatment or care should a healthy tree receive in the summer months?

Answer: It’s always wise to lean on an experienced arborist for this guidance. It depends on the species but most need to be pruned. Younger and smaller trees often require more water.

Question: What is your team currently working on?

Answer: We’re heavily researching the benefits and the costs of trees and vegetation in Denver. We’re interested in a wide suite of things, ranging from heat mitigation to storm-water mitigation to rainfall interception to socioeconomic factors like crime and property values. The goal is to provide the tools to help ensure that the benefits of trees are fairly and equitably distributed throughout Denver.

Question: What are you up to in your own yard?

Answer: I recently hired a landscape architect to help me bring my vision to life. Up until now I had just been maintaining my yard and not thinking outside the box. Now I want to introduce more native, low-water plants and interesting landscape features. Hopefully, there will be a lot less space that requires irrigation. I’m excited about it!

Apply for a Free Tree

Do you have room for a new tree? Being a Smart Ash means that we plant new trees before losing established ones to the EAB. In fact, Denver Parks and Recreation wants to help you plant a tree. If you have room in the public right-of-way next to your property, you may even be eligible to have a free tree planted for you! Learn more here.

Denver’s Ash Trees Ready for Second Treatment

If you were out and about in Denver in 2016, you likely saw hundreds of ash trees wrapped in plastic that said, “Be A Smart Ash.” Denver Parks & Recreation’s Office of the City Forester treated public right-of-way ash trees adjacent to private properties throughout the city to prevent an Emerald Ash Borer (EAB) infestation. Although the treatment of ash trees by a tree care professional is 90% effective, treatment must be repeated every two to three years to remain successful. With this in mind, we will be treating these trees again.

Healthy ash trees that are 12 inches and larger in diameter at four feet off the ground were put on a list for potential treatment. From this list, a number of trees were randomly selected in each neighborhood to be treated in either 2016, 2017 or 2018.  Because trees require treatment every two to three years, we’re revisiting the 2016 trees now. Every ash tree scheduled to be treated was examined by a city arboreal inspector to determine if it was a good candidate for treatment.

EAB is a destructive pest that kills ash trees and poses safety and financial risks. However, this treatment, when done in advance, is more than 90% effective in preventing tree death.

photo courtesy of @watersa on Instagram

Risks of avoiding treatment Include:

  • Ash trees killed by EAB become very brittle and can break easily. Dead ash trees are dangerous to you and your neighbors and can be expensive to remove.
  • You are responsible for removing infested or dead EAB trees on and adjacent to your property, so treat your tree!
  • EAB has the potential to destroy Metro Denver’s 1.45 million ash trees – roughly 1/6 of Denver’s trees – significantly impacting our precious tree canopy.

Trees are critical for our overall quality of life. They produce oxygen, reduce smog, cool our neighborhoods and homes and increase our property values. Join our effort to save Denver’s ash trees by talking to your neighbors, friends and co-workers about EAB and encourage people to visit for more information. To see which ash trees are being treated and those that are candidates for future treatment, visit

Removing and Replacing Your Ash Tree

One of the four recommended treatment options for a tree infected with emerald ash borer (EAB) is to remove the tree and replace it with another species that can grow well in Colorado. Though the EAB has yet to be found and confirmed in the Denver Metro Area, the removal and replacement option is both cost-effective and convenient for homeowners.

Step one is removal of the ash tree and step two is replacement. While it sounds easy enough, hiring a tree professional will make this process significantly easier. There are many tree professionals that are licensed by the Office of the City Forester who can lend a hand.

We connected with the wise folks at Denver Parks & Rec to ask for their recommendations for tree replacements. We asked for suggestions for species that will easily thrive in our local climate when provided adequate care. Drumroll, please…

Smaller Trees | Less than 30-feet high

Amur Maackia – photo courtesy of Oakland Nurseries
Any redbud – photo courtesy of Arbor Day Foundation
John Pair maple – photo courtesy of Iowa City Landscaping
Peking lilac – photo courtesy of Forestfarm
Three-flower maple – photo courtesy of Heritage Seedlings

Shade Trees | 30-feet and taller

Espresso Kentucky coffeetree (without seeds) – photo courtesy of Your Garden Sanctuary
Prairie expedition elm – photo courtesy of Laidback Gardener
Tuliptree – photo courtesy of Tennessee Wholesale Nursery

Trees in the approved list are those which, given proper and consistent maintenance including supplemental irrigation, proper pruning, and avoidance of chemical contaminants, will be assets to Denver’s beautiful urban canopy. You can also explore the approved list in its entirety.