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Trees are our first line of defense when it comes to combatting the urban phenomenon of the Heat Island. No, Heat Island isn’t a new reality show. Although it is a reality.  

What is a Heat Island? 

The easy explanation by NASA’s Climate Kids is that an urban heat island occurs when a city experiences much warmer temperatures than nearby rural areas. The difference in temperature between urban and less-developed rural areas has to do with how well the surfaces in each environment absorb and hold heat. Cities tend to have more impervious surfaces, such as buildings, sidewalks, roads and patios, which hold more heat.  

Cities throughout the world are experiencing urban heat islands, and Denver is no different. Take a look at Denver’s Heat Island map:

This map uses impervious surfaces as a proxy for heat. In this map, the darker colors represent hotter areas 

…and now take a look at the map of the city’s trees:  

In this map, the darker the color, the higher percentage of tree canopy cover there is. 

See how they are essentially opposites of each other? Meaning, the places with lower heat vulnerability are also the places with the highest tree canopy coverage. It’s clear to see that trees make a difference.  

Now that we know it’s this easy, what can we do? You guessed it.  

Plant. More. Trees.   

Now you may be asking, what is it about trees? It can’t just be the shade they offer. What else is taking place? It’s a great question with only a halfway-surprising answer. Because a lot of it does have to do with the shade they provide and the sunlight they deflect. But there is also something else happening that helps trees cool their surrounding area… 

Evapotranspiration.  

Over at USGS.gov (United States Geological Survey), a science bureau within the United States Department of the Interior, there is a simple illustration that says it all: Evapotranspiration simply describes how water working through the tree ends up cooling the air around it. Water (precipitation) comes in through the roots and is released as vapor (transpiration). And, as an added bonus, the soil releases even more water to cool the air down low (evaporation).

So, one of the best ways we can combat heat islands is by planting more trees. Two ways you can do that: apply for a free tree in your public right-of-way and plant more trees downtown.  

All of the above combined results in a drop in temps and a happier, healthier – and much cooler –community. If you’ve read all the way through and you still want to learn more, check out this PBS interview from 2021, featuring Denver’s own City Forester Mike Swanson talking about Heat Islands.   

If water is life, then mulch is a lifesaver! When we talk about tree care in the summertime, we are mostly talking about the one-two punch of watering and mulching. Below you’ll find a round-up of some simple tips for summertime tree care.  

BE WATER-WISE 

Before you start watering, make a plan based on the trees you have. Answering these questions can help you make a watering plan that works. 

• How old are your trees?  
• How much water will they need?  
• How do you tell if you’ve watered enough?  
• When should you water?  
• When should you not?  

How can I check if my tree needs to be watered?  

Easy! Feel it with your hand. Dig down into the soil a few inches and squeeze with your hand to feel if it is wet, dry or somewhere in-between.  

How much water do I use?  

If your tree is young and still establishing, it will need a little more TLC, so shoot for 15-20 gallons, every 2-3 days. After your tree becomes established (it usually takes a two-inch caliper tree 2-3 years to establish), you can reduce the watering frequency. However, we still recommend following the 10 gallons of water per inch caliper of the tree, watering when we experience long periods without precipitation and throughout the winter. Allow soil to dry between waterings and continue to monitor your soil moisture to determine when your tree needs to be watered.  

How can I make sure water reaches deeper into the soil? 

A simple hose is the most basic tool needed to water your tree, but soaker hoses, soft spray nozzles and soil needles can help break through the soil surface. Most absorbing tree roots are found in the first 12-inches of soil depth; apply water slowly so it has time to absorb into the soil and reach these vital roots.    

How can I tell how much water my tree is getting through my hose or irrigation system? 

You can determine your hose’s output by taking a container of known quantity, like a gallon jug or five-gallon bucket, and setting a timer for how long it takes the hose to fill up the container at a trickle. Then you can calculate how long you should trickle your hose to achieve a certain quantity of watering. Irrigation systems themselves generally do not provide enough water for young trees during their establishment phase, so plan on giving supplemental water as well and checking the soil moisture regularly.  

What else?  

As travel begins to pick back up and Denver residents start hitting the road, don’t forget to make a tree care plan while you’re gone. When possible, ask a neighbor, family member or friend if they’ll water for you and you can offer to do the same for them. And remember to check with your utility department for preferred times, right-day usage and other potential temporary changes.   

MULCH-MUSTS 

Mulch: it does so much! Keeping a mulch ring around young trees keeps them safe from mower and string trimmer damage, which can be an easy point of entry for pests and disease. It also holds moisture in and protects against temperature extremes. It decomposes and adds nutrients to the soil, too! 

Here’s how it’s done:  

Apply wood chips, bark, or other organic mulch 2 to 4 inches deep in a ring around the base of the tree, but at least 3 inches away from its trunk. Mulch reduces soil evaporation, improves water absorption, insulates against temperature extremes and can break down over time, adding necessary nutrients to the soil. Make sure to replenish your mulch as necessary, at least a few times a year.  

We said MULCH, not rocks or grass.  

Rocks as “mulch” are bad for trees. They retain heat, causing the soil temperature to rise and that increases evaporation. Plus, rocks don’t provide essential nutrients and can actually change the soil chemistry, which can be unfavorable for many trees. They’re not all bad, though – rocks and xeriscape lawns can be a way to conserve water in other areas of your garden, but it is essential that the area with your tree roots is covered with mulch or other organic material.  

For different reasons, grass around the base of the trunk is also not recommended. Why? Because grass is greedy. It competes with young trees for moisture and nutrients in an already water-scarce climate. Plus, having grass close to your tree increases the risk of mechanical damage to the trunk from lawn mowers or string trimmers.  

Vertical Mulching 

For those of you ready to graduate from Mulch 101 and really dig into the topic, we present vertical mulching. Vertical mulching can alleviate soil compaction, which jeopardizes tree health and is commonly found in Denver in areas with clay soil or recent construction. Vertical mulching improves soil conditions and allows for deeper water penetration. Yep – it’s all related. It’s accomplished by removing columns of poor-quality soil around the tree in a radial or grid pattern, and then filling those columns with compost. You can watch this video for more information.  

TREE VITALITY = EARTH VITALITY  

We hope that all these tips help you determine how much water each of your trees need, what type of mulch to use and why it all matters. Because when done correctly, these steps can increase tree vitality, which can help your trees stay healthy year-round and keep our urban canopy in great shape for the future. 

We interrupt our regularly scheduled how-to blog posts to bring you information about two tree-riffic events happening in Denver later this week: The International Society of Arboriculture’s Rocky Mountain Chapter Tree Climbing Competition and Earth Day Remastered.  

The Be A Smart Ash team will have a booth at both events, so we hope you stop by and say “hi”. And don’t worry, we’ll have plenty of how-to information on hand.   

The International Society of Arboriculture’s Rocky Mountain Chapter Tree Climbing Competition  

When: June 24-26, 2022  

Where: City of Nairobi Park, 3500 Cook St., Denver, CO 80205 

What: The International Society of Arboriculture’s (ISA) Rocky Mountain Chapter Tree Climbing Competition is happening this weekend, and it’s just what it sounds like. But it’s not for kiddos – this one’s for the pros. Working arborists in the Rocky Mountain region from Colorado, New Mexico, Wyoming and Montana will be demonstrating climbs, and exchanging ideas for new techniques and equipment, as well as best practices for safe work, all in an atmosphere of friendly competition.  

From The International Society of Arboriculture’s website:  

Professional tree climbing competitions are held around the world to provide a platform for arborists to learn about the latest in climbing techniques and innovations in equipment. They showcase the highest level of professional skills and safety, providing a competitive learning environment for those working in the industry. 

The competitions simulate working conditions of arborists in the field. Male and female competitors perform five different events during preliminary rounds. Each event tests a competitor’s ability to professionally and safely maneuver in a tree while performing work-related tree-care tasks in a timely manner. 

Competitive tree climbing also introduces the public to the skills professional tree climbing arborists must use for safe, professional tree work. 

View the complete schedule if you want to stop by and watch the action. The winners of this event will go on to represent ISA Rocky Mountain Chapter at the North American Tree Climbing Championship as well as the annual International Tree Climbing Championship. 

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Earth Day Remastered 

Come celebrate our commitment to a healthy planet – it’s free and open to the public!  

When: Sunday, June 26, 12-4 p.m. 

Where: Washington Park, South Meadow near Franklin and Mississippi 

What: Join District 6 Councilman Paul Kashmann and the District 6 team for Earth Day Remastered, a free community event “Celebrating Our Commitment to a Healthy Planet.” The afternoon will feature music, exhibitors displaying products and services to help reduce your carbon footprint and keynote speakers committed to climate action. In addition to Councilman Kashmann, Earth Day Remastered is sponsored by the University of Denver and Love My Air Denver. 

For more information, visit the website.   

Since 1872, this “day” has been a celebration of trees. Its birthplace is Nebraska City, NE, but it took root on both coasts and eventually propagated worldwide. Today, this holiday remains an expression of how we joyfully celebrate the planting of trees. According to the Arbor Day Foundation, Arbor Day is a day for people to come together in recognition of the wonders of trees. Communities, schools, businesses, and individuals alike join together to plant commemorative trees, hand out free trees, provide education on tree planting and care, and celebrate all the benefits trees provide.

Now, this is a holiday we can all get behind!

While it’s 150 years strong – older than most of the trees we see every day – it is surprisingly not always easy to track down this holiday on the calendar. So, when is Arbor Day, anyway? National Arbor Day is the last Friday in April, while here in Colorado, we celebrate Arbor Day on the third Friday in April. But take a look at the map and you’ll see that it changes from state to state.

For example, Arbor Day is celebrated as early as January in Louisiana and in November in Texas… and even as late as December in South Carolina. A quick internet search will tell you that the time to celebrate is flexible, and that is based on the best times to plant trees in that area.

#DenverLovesTress Photo Contest

In May, and in honor of this year-long holiday, we decided to branch out and launch a little social media competition here in Denver. The weekly winners received an Ace Hardware gift certificate, so they could celebrate Arbor Day by planting a tree when it’s convenient for them.

Look at the compilation of photos from fellow tree lovers and you’ll understand why we were stumped by which submissions to choose. It’s pretty tough to take a bad picture of something so rooted in natural beauty. But we managed to be-leaf in ourselves and eventually selected the following winners (and we’re sharing some of the runners-up, as well).

With that, we wish you a Happy Arbor Day, Denver! And remember: The best time to plant a tree may be 20 years ago, but the second-best time is today.

Contest-winning images:

…and while these didn’t make the podium, we still think they’re worth sharing.

Other submissions from the contest:

So, you want to plant a tree?

It’s April in the Mile High City, and spring has sprung – or at least it’s about to – and with spring so close you can almost smell it, the time is right for successful Denver tree planting. By choosing to plant a tree in Denver, you’re choosing to contribute to Denver’s urban canopy. Here is everything you need to know about planting a new tree.

What type of tree should you plant?

The first thing you might be thinking about is what you want to plant. There are so many trees available, it can get overwhelming. And it’s important to plant a tree that will grow and thrive in its surroundings. Luckily, we’ve compiled a list of some the best trees to plant in Denver, based on our unique climate.

You’ve picked your tree. Now what?

There are a few ways you can go about planting your new tree:

• Choose a Denver landscape professional or hire an arborist to plant your new tree.
• Get your hands dirty and plant your tree yourself with these step-by-step instructions from Colorado State University.  
• If you’re interested in planting a tree in the public right-of-way adjacent to your Denver property, apply for a free tree through Denver’s Office of the City Forester. If your request is approved, we’ll deliver and plant the tree for you!

Keep in mind that regardless of which option you choose, it doesn’t happen overnight – it takes about 2-3 years of ideal conditions for a 2-inch caliper tree to establish. Although it’s a commitment, it’s well worth it. 

When to plant your new tree?

Now! ‘Tis the season for tree planting in Denver. Know why? Milder temperatures are less harsh on young trees when they are first getting established. Another tip: planting trees that are currently dormant or “sleeping,” like they are this time of year, also reduces the tree’s stress when it’s dug from the tree nursery to be transported and replanted in your yard.

Where to plant your 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 the same spacing guidelines on private property.

Tree vitality tips to ensure your new tree survives

You did the research, you made a commitment and you planted a tree. Now, how can you ensure your tree continues to grow and thrive for many years to come? Here are Denver’s Office of the City Forester’s top tips for tree vitality:

1. Check soil moisture and water more frequently during periods of drought and prolonged heat, but be careful not to overwater.
   
   
2. Keep a mulch ring around young trees.
   • This keeps tree safe from mower and string trimmer damage, which can be an entry for pests and disease. It also holds moisture in and protects against temperature extremes. It decomposes and adds nutrients to the soil, too! 
     
3. Staking is usually unnecessary unless the tree has a severe lean that needs to be corrected or is exposed to high winds, such as in Northeast Denver, where the winds blow a little harder.  
   
   
4. Until your tree is established, you may not see much growth from the crown—that’s normal!
   
   
5. Fertilizing is not recommended.
   • If you want to improve soil conditions, try vertical mulching or adding compost to entire area.
     
6. Healthy trees can fight off many pests and small wounds without intervention.
   • Keep your tree healthy by making sure it gets enough water all year round.
     
7. Suckers, or young stems sprouting from the base of your tree’s trunk, indicate a stressed tree, so prune them back and make sure you’re not over or under watering.
   
   
8. If you need additional help caring for your tree, you can hire an experienced arborist to help with pruning, tree removal, emergency tree care, planting and more.

They say the best time to plant a tree is 20 years ago. The second-best time? Today. So, invest in your own future, your kids’ and grandkids’ future – and the future of the city, too – because when it comes to planting a tree or two in Denver, it benefits us all for generations to come.

Part of what makes Denver a great place to live is its altitude and climate. But did you know that because Denver is located in the high plains, our ecosystem is home to very few large, deciduous tree species? These are the trees that make our city shady and provide us with benefits like improving air quality and combatting heat islands. In this blog post, we’ll give you tips on the best trees to plant for Denver’s climate and how these recommendations help our urban canopy.

While Denver’s Office of the City Forester wants all Denver residents and property owners to plant more trees and help grow our urban tree canopy, some trees are simply better equipped than others to survive in our unique climate. And while it’s important to plant new trees here in the city, it’s even more important that those trees survive. To make it easy for you to choose an appropriate tree, we’re providing this handy list of the what’s and why’s for our climate, and which trees are not only best suited to survive, but to thrive, in Denver. 

What makes Denver’s climate challenging for trees?

There are two primary factors that make it challenging for some large trees to thrive in Denver:

1. Soil: Colorado soils generally have adequate to high iron levels, as evidenced by the many red soils and rock formations. However, Colorado soils are mostly alkaline, causing most of the iron to be in a form not usable by plants.
   
2. Water: Denver is also a semi-arid climate, meaning we get some precipitation but not much. Young trees need consistent watering here to establish and thrive, especially during periods of drought and high temperatures.

Which are the best trees to plant in Denver?

Which trees are most likely to thrive in our climate? Here’s our top 8 trees to plant:

1. Kentucky Coffeetree (large): The Kentucky Coffeetree adapts well to a wide range of soils and urban environments. Its drought and pollution tolerance makes it a great choice for city living, particularly in Denver’s arid climate.
   
2. Common Hackberry (large): A shady tree with a widespread crown that turns yellow in the fall, the Common Hackberry is native to North America and withstands winters sturdily.
   
3. Northern Catalpa (large): The Northern Catalpa is drought tolerant and grows well in alkaline soil with its large heart-shaped leaves and white flowers in the springtime.
   
4. Bur Oak (large): One of the oak tree varieties, the Bur Oak’s most prominent feature is the acorns it grows. It’s often planted in Colorado where it thrives in our climate and soil.
   
5. Turkish Filbert (medium): Tolerant of a wide variety of soil conditions, the Turkish Filbert’s leaves turn yellow in the fall and may produce small nuts.
   
6. Thornless Cockspur Hawthorn (small): Covered in white flowers in the spring and persistent fruit through the winter, this small tree grows horizontal branches and provides food for birds.
   
7. Autumn Brilliance Serviceberry (small): Known for its pretty white flowers in the spring, this small tree is a relatively low maintenance tree and great for attracting birds and bees to your yard.
   
8. Amur Maackia (small): A shapely, small tree with beautiful red fall color, the Amur Maackia is very hardy and adaptable to Denver’s climate.

All of these trees are commonly planted through our free tree program. If you have room in your public right-of-way, an arborist will visit your site and make a recommendation for what kind of tree may thrive there.

The Front Range Tree Recommendation List, developed by Colorado State University, is also a helpful resource for seeing how various tree species rate based on the factors of our area. Keep in mind this publication was released in 2010, so while ash trees are included, because of the threat of emerald ash borer, the planting of ash trees is no longer permitted.

What trees don’t do as well here? 

Although beautiful trees that everyone wants on their properties, a few trees shouldn’t be planted in Denver because they’re less likely to thrive in our climate. Here are two to avoid:  

1. Maples: Maple trees include a number of species that are known for their vibrant colors and notable leaves. However, maples are susceptible to chlorosis, or leaf yellowing due to insufficient iron. This happens often to maples here in Denver due to our alkaline soil. Maples also have thin bark which makes young trees especially susceptible to damage due to sunscald. And finally, maples are not very drought tolerant, which can be a challenge in our semi-arid climate.
2. Aspens: Aspens are native to higher altitudes, which is why we all make trips to the mountains to see their fall color. They thrive in areas that aren’t as hot and have different soil conditions than we have here in Denver. Aspens in the metro area are more susceptible to pests and diseases and have inherently shorter lives.

How do I navigate the conversation with my landscape professional on what to plant?

It’s always best to show up informed to a meeting with your landscape professional. Bring this article, the Front Range Recommended Tree List or the Forestry-Approved Street Tree List to help guide you. In addition to coming informed on what trees are best for Denver’s climate, you should also show up with an idea of what tree values are most important to you:

• Shade – look to large species with dense canopy
• Aesthetic – look for species that flower/you enjoy the look of 
• Wildlife – look for species that provide fruit/flowers for wildlife/pollinators 
• Xeriscape – look for more drought tolerant species 
• Space – limited space requires ornamental or medium sized trees 

Make sure your landscape professional is considering sunlight, soil, irrigation, surrounding tree diversity and distance from structures and other trees to pick the best tree for your location.

Above all, just remember the mantra, “right tree, right place.”

Lexi Brewer, Urban Forestry Operations Assistant, City and County of Denver

This past year was a busy one for Denver’s Office of the City Forester, as we provided free trees, education and resources to the community. Not only did we continue educating and enlisting the help of Denver residents to identify, treat and replace ash trees as part of our Emerald Ash Borer (EAB) prevention efforts, our work extended even further into the community as we implemented additional programs available to residents like planting free trees, the GAP tree removal and replacement plan and the Forestry Neighborhood Initiative in southwest Denver.

Read on for a recap of what we accomplished this year and how you can apply to receive a free tree in 2022.  

2021 by the numbers

Council District	# of trees planted
1	213
2	30
3	73
4	38
5	57
6	119
7	278
8	230
9	140
10	88
11	92
Total	1,361

Why we plant trees in Denver

As part of the Be A Smart Ash campaign, we’ve been focused on replacing ash trees as a defense against EAB. Since 1 in 6 trees in Denver is an ash, when EAB is discovered here, it will leave gaps in our beautiful but fragile tree canopy. Therefore, we’ve been proactively combatting these inevitable gaps with the addition of new and diverse tree species.

In addition to defending against EAB, trees are important for the health of Denver’s residents, particularly in the summer. Trees cool and clean the air, provide shade and slow down rainfall. In fact, American Forests estimates that nationwide, city trees prevent approximately 1,200 heat-related deaths and countless heat-related illnesses annually. Learn more from Denver City Forester Mike Swanson about how trees combat extreme heat in Denver’s historically underserved neighborhoods.

Why the need for target neighborhoods

Denver’s Office of the City Forester works with community partners and researchers to prioritize areas of the city that could benefit the most from free tree services. Learn more about how we prioritize tree planting areas in neighborhoods across the city depending on canopy cover and household income.

Where to apply for free trees in 2022

Tree plantings, removals and prunes this year were all part of three different programs that together served our Denver community. Learn more about these programs and how you can access their services in 2022:

• Free Trees Program: Since the launch of the Be A Smart Ash program, the Office of the City Forester has planted more than 10,000 free trees for Denver property owners in the public right-of-way. It’s easy to apply for a free tree online and if you qualify, you’ll receive tree planting support and instructions on how to care for your tree.
• GAP Program: The Ash Tree GAP Removal & Replacement Program removes and replaces smaller, poor-condition ash trees in the public right-of-way throughout the city. In 2021, 227 trees were removed, and 165 trees were planted as part of the GAP Program.
• Forestry Neighborhood Initiative: The Denver Forestry Neighborhood Initiative focuses on pruning or removing trees that pose a risk to public safety, as well as planting new trees when space allows. In 2021, this initiative oversaw the planting of 208 trees, as well as the removal of 110 trees, 13 tree prunings, 2 stump grinds and the watering of 167 trees. Property owners who qualify for a free tree or for tree maintenance will receive a letter and a postcard in the mail with information on which service(s) they qualify for and how to claim them. However, every property owner in Denver is eligible for a free tree if they think they have space in their public right-of-way.

We look forward to another year of partnering with our Denver community to protect our tree canopy and improve the health and wellness of residents through trees! For more information about any of these programs, email tree@denvergov.org or call the Denver Office of the City Forester at 720-913-0651.

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