Seasonal Tree Care Needs

Overview

The environment in which a tree is planted impacts its ability to thrive. The city’s regulations prohibit the following, which negatively impact your tree’s health:

• Mulching with rocks because they retain heat, raise the soil temperature, increase evaporation and lead to stressed trees that require even more water. 

• Planting trees too close to metal grates or concrete.
• Planting turf grass at the base of the trunk, as it robs young trees of moisture and nutrients and increases the risk of mechanical damage from lawn machinery. 

When the seasons change, so do your tree’s needs, although a watered-in ring of proper mulch around the base of your tree is a best practice for Denver trees in any season. Curious about seasonal tips? The following best practices will keep you on track for seasonal tree care.

As always, if you have specific questions about tree care, reach out to Denver’s Office of the City Forester at forestry@denvergov.org or 720-913-0651.

Spring (March – May)

Watering, pruning and other light maintenance prepares trees for the growing season. This is the time to schedule irrigation turn-on and continue to water weekly with 10-15 gallons, and more when the weather is dry, windy or if there are prolonged periods of drought and sunshine.

Pruning

• This is your last opportunity for branch pruning on elms and fruit trees. Do it before buds and leaves appear to prevent spreading disease.
• It is recommended to hire a tree care professional for pruning trees that may have disease to prevent further spread. If you attempt to prune yourself, make sure to clean pruning tools the right way: Wipe the cutting surface thoroughly between each cut with disinfectant or rubbing alcohol, wearing gloves for safety. Make your pruning cut well below the location on the limb or branch that appears to be diseased.
• Avoid pruning thin bark trees such as maples because it can cause sap oozing, sometimes referred to as “bleeding.” If your tree is exhibiting signs of oozing, contact an arborist as it could be a sign of insect or disease activity.
• Before shearing or pruning evergreens, look for bird nesting activity so you don’t scare birds away.
• Check that your tools are ready, in safe working order, clean and sharp. It is recommended to wipe down the cutting blades of your pruning tools with a rag and disinfectant like bleach or rubbing alcohol to reduce chances of passing on any disease that may be lingering.
• Check clearance and prune branches over streets/alleys (13.5’) and sidewalks (8’), as well as vegetation protruding into walkways.

Raking

• Raking in the early spring has multiple benefits:
  • Removes possible (re)contamination of new emerging leaves.
  • Provides better airflow to the base of plants and the soil.
  • Allows turf areas to dry out, preventing some diseases from developing. If possible, wait for the native bee emergence before removing leaves, dead plant stalks/branches or adding mulch.
  • If leaves are not diseased, they can be mulched or raked into smaller pieces to incorporate organic matter back into the soil. Learn more about why it’s important to leave a few leaves behind.

Fertilizing

• It’s recommended to have a soil test completed prior to adding any fertilizer so that you are not adding unnecessary “substances” to the soil, contributing to pesticide waste to the water supply or wasting your hard-earned dollars.
• If you have not had a soil test and notice poor plant and turf performance, try adding a generic fertilizer with a 10-10-10 ratio. Always follow the label.

Summer (June – August)

The two most important summertime tree care activities are watering and mulching.

Watering

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

• How old are your trees?  
• What size are they?
• What species are they?
• How much water will they need or what is the tree species’ needs? For example, is the tree a low water-use tree or other? 
• Is your tree in an area that receives more or less water than other areas?
• Does water absorb into the soil quickly or slowly?

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.

Water established trees once a week with 10-15 gallons of water, and more when the weather is dry, windy or during prolonged periods of drought. To test for moisture, dig down into the soil a few inches and squeeze with your hand to feel if it is wet, dry or somewhere in-between. Allow soil to dry between waterings and regularly monitor your soil moisture to determine when your tree needs to be watered.  

Check your outdoor watering rules but generally they are in effect May 1 to Oct. 1. There may be specific requirements concerning the below, but the following are recommended:

• Water during cooler times of the day – lawn watering is NOT allowed between 10 a.m. and 6 p.m.
• Water no more than three days per week.
• Do not allow water to pool in gutters, streets and alleys.
• Do not waste water by letting it spray on concrete and asphalt.
• Repair leaking sprinkler systems within 10 days.
• Do not irrigate while it is raining or during high winds.
• Use a hose nozzle with a shut-off valve when washing your car.
• Learn about Denver’s rules for watering. 

Go Deep

Ensure your water reaches deep 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 deeply absorb into the soil. This will help to establish deeper roots for new trees to protect from the sun’s heat and competition with turf grass and reach the vital absorbing roots on established trees.   

Check Output

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 alone generally do not provide enough water for young trees during their establishment phase, so plan on supplementing water and checking the soil moisture regularly.  

Contingency Care

Remember your trees in your summer travel plans. Ask a neighbor, family member or friend to tree-sit. And remember to stay current with Denver Water’s rules for outdoor usage.  

Mulch and turf grass

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 6 inches away from its trunk. Mulch reduces soil evaporation, improves water absorption, insulates against temperature extremes and can break down over time, adding necessary organic matter and nutrients to the soil. Make sure to replenish your mulch as necessary.  

Do not mulch with rocks, which retain heat, raise the soil temperature and increase evaporation. Plus, rocks don’t provide essential nutrients and can actually change the soil chemistry, which can be unfavorable for many trees. 

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 

Vertical mulching removes columns of poor-quality soil around the tree in a radial or grid pattern and fills those columns with compost. Vertical mulching improves soil conditions by adding organic matter, providing better air flow and allowing deeper water penetration and it’s commonly used in Denver in areas with clay soil or recent construction. You can watch this video for more information.  

Fall (September – November)

Fall is also a good time to plant new trees, as summer has left behind healthy soil and there are still plenty of sunny days before the first freeze to get roots growing. The best time to plant your fall tree is generally through October, but at minimum when nighttime temperatures are consistently above freezing and prior to the ground freezing. Planting a dormant tree can ensure that all the tree’s energy goes to establishing roots, ahead of the spring growth flush. A mulch ring around your tree holds moisture in and protects against temperature extremes. It also decomposes and adds nutrients to the soil, too. Tree diversity is the healthiest way to combat future pests and disease in our urban forest.

Water

Water established trees twice a month with 10-15 gallons of water if the ground isn’t frozen. This is best done when the temperature is above freezing and ideally around 40 degrees and at mid-day to allow water to soak in in case the ground freezes at night. Continue to check the moisture level of the root ball (the main mass of roots directly beneath the trunk), especially during long periods without snow cover. 

Winter (December – February)

Proper tree care includes winter months for tree health and survival. Trees become dormant as they prepare for colder temperatures. Watering dormant trees during dry periods (two or more weeks without snow cover) helps them remain in good health into spring.

Applying mulch around the tree holds moisture and protects against extreme temperatures. It decomposes, adding organic matter and nutrients to the soil, too. Trees with smooth bark, especially on the southern/southwestern face, are subject to a condition called sun scald. To avoid this, the tree can be wrapped using materials like tree wrap or butcher paper. It’s waterproof and blocks some of the energy that the sun produces. CSU Extension shows you how to properly wrap a tree for the winter. 

Some preparation for winter begins in the fall, such as winterizing irrigation systems. For additional information, read more from the Colorado State Forest Service on winter tree watering.

Monitoring snow fall for accumulating snow on branches is a good idea, as heavy snow, especially with windy conditions, can weigh down branches causing them to break. When it is safe to do so, gently shake or tap limbs using an upward motion with the soft end of a broom to knock the snow off. Always be aware of limbs in contact with power lines/sources and conditions if branches are breaking to avoid injury.

Monitor trees to ensure there is no oozing, staining, abnormal discoloration of bark, die back of limbs/branches, injury to the trunk from boring insects or birds or mower damage.

Watering Routine

Water established trees twice a month with 10-15 gallons of water if the ground isn’t frozen. This is best done when the temperature is above freezing and ideally around 40 degrees and at mid-day to allow water to soak in before the ground freezes at night. Continue to check the moisture level of the root ball (the main mass of roots directly beneath the trunk), especially during long periods without snow cover. 

Pruning

Certain trees, including American elm and fruit trees in the rose family, should only be pruned in winter – while dormant – to reduce the spread of disease. Avoid pruning trees with thin bark if possible as sap flow could leave an unpleasant looking stain on lighter colored bark. If you are pruning something you can’t reach or can’t make a proper cut, it’s advised that you hire a tree care professional since they use specialized equipment, follow strict safety guidelines and have training and expertise. The City of Denver requires tree contractors to be licensed and insured, and we’re pleased to provide a list of licensed tree contractors for your convenience.