A 20-inch diameter ash tree needs about 200 gallons per watering using CSU’s rule of thumb: 10 gallons per inch of trunk diameter. During a hot, dry Denver summer, that usually means about 200 gallons every 1–2 weeks, adjusted for rainfall, soil, and tree stress. – CSU Extension
Mature Trees Need Deep Watering, Not Frequent Watering
Denver homeowners are adjusting to water restrictions enacted by Denver Water to conserve during this year’s unprecedented drought. People are allowed to water their lawns twice per week, rather than three times, meaning yard trees get one-third less water. The good news is that Denver Water allows homeowners to water trees, shrubs, and perennial gardens outside these restrictions. The challenge is understanding how much deep root watering is needed to fill the gap to keep mature and newly planted yard trees healthy.
Why Trees Are Affected by Lawn Watering Restrictions
Oaks, Maples, Honeylocust, and Ash trees planted in yard landscapes have spent decades growing in irrigated bluegrass lawns. Over time, these trees develop shallow root systems that take advantage of moisture supplied by lawn sprinklers. Why go deep if you don’t have to? When irrigation is reduced, trees suddenly receive less water than they are accustomed to. Fortunately, May and June have not been too hot. But if July and August temperatures reach the 90s and 100s, homeowners might want to consider deep-root feeding and watering their trees. Although mature trees are more resilient than grass, prolonged drought can weaken them, making them more susceptible to insects and diseases.
Denver’s Clay Soil Creates Unique Challenges
Denver gardeners are well aware of the challenges of our clay soils. While clay soils do retain moisture longer than sandy ones, they also create problems during drought. In hot weather, clay soils can become rock hard. Water simply runs off before reaching the root zone. In baked soils, deep-root watering may be needed to deliver moisture where it is needed. Homeowner should put mulch around all their yard trees to maintain consistent soil moisture.
Newly Planted Trees Need Extra Attention
Trees planted within the last three years are considered saplings and vulnerable to drought stress because they have underdeveloped root systems. Their roots cannot reach far-away ground moisture. A young tree can suffer permanent damage after only a few weeks of severe drought stress.
If it gets hot in July and August this year, homeowners will most likely have to get used to brown lawns. If they are lucky, their bluegrass turf will go dormant and come back next year. Trees do not have this option, and homeowners should consider supplemental watering to keep their trees healthy this summer. Most homeowners do not have equipment or time to correctly apply hundreds of gallons of water to mature trees. They should call a Denver tree service company with an ISA-certified arborist on staff, such as Ross Tree. Our arborist can determine how much water each tree needs and recommend a summer and fall watering and feeding schedule.
Which Trees Are Most Vulnerable?
Not all trees respond to drought the same way.
High-Risk Trees
- Silver Maple
- Autumn Blaze Maple
- Freeman Maple cultivars
- Birchs
- Colorado Blue Spruce
- Ash Trees treated for EAB
- Cottonwoods
- Newly planted trees of any species
Cottonwoods are among Denver’s thirstiest trees. Mature specimens can consume hundreds of gallons of water on hot summer days. Reduced irrigation often accelerates drought stress, branch failure, and overall decline.
Silver Maple is the second most exposed. Ross Tree’s blog content flags these trees as already prone to fungal infections and hollowing out. Drought stress suppresses the tree’s natural defenses, and Silver Maples planted in the older Denver neighborhoods — Congress Park, Washington Park, Bonnie Brae, and University Park — are already at or past peak age.
Ash trees under EAB treatment are particularly vulnerable. The systemic trunk injection treatment Ross Tree recommends depends on active transpiration to distribute insecticide through the vascular system. A tree too drought-stressed to transpire properly will not distribute the treatment effectively, compromising the entire treatment investment. All Ash in Denver should be treated for EAB.
Moderate-Risk Trees
These trees generally tolerate drought better but can still experience stress:
- Honeylocust
- Linden
- Austrian Pine
Honeylocust becomes susceptible to Thyronectria canker under consecutive drought years. Once established, there is no cure, so water stress this summer could trigger a fungal infection that shows up as cankers over the next 1–2 seasons.
Norway Maple, despite its aggressive reputation, has shallow roots that can dry out quickly. These trees suffer visibly when stressed and, if not treated promptly, may require removal.
Linden trees are moderate risk.
More Drought-Tolerant Trees
These species are naturally adapted to drier conditions:
- Bur Oak
- Gambel Oak
- Hackberry
- Kentucky Coffeetree
- Catalpa
Ross Tree often recommends these trees for planting because they are drought-tolerant, well-adapted to alkaline soils, and deep-rooted enough to access subsoil moisture that sprinkler systems do not reach. However, with these restrictions in place, if the summer turns hot, all these trees would benefit from monthly deep root watering.
Common Signs of Drought Stress
Drought stress can weaken trees and make them more vulnerable to a variety of health problems, so watering alone is not always enough to restore tree health. Trees experiencing prolonged drought conditions are often more susceptible to iron chlorosis, wood-boring insects, fungal diseases, secondary pest infestations, and root decline. Homeowners should monitor trees closely throughout the summer. Warning signs include:
- Thinning canopies
- Leaves turning yellow or brown
- Scorched leaf margins
- Premature leaf drop
- Branch dieback
- Reduced annual growth
- Increased insect activity
When these symptoms appear, the underlying issue may be more complex than drought stress alone. An ISA Certified Arborist can evaluate the tree, identify the root cause of the problem, and determine whether improved watering practices will be sufficient or if additional treatments are needed to preserve the tree’s health and longevity. The earlier drought stress is identified, the easier it is to correct.
Following CSU’s 10 gallons per inch of trunk diameter rule, most homeowners do not have equipment or time to water mature trees. Denver property owners should not wait until branches begin dying back; by the time visible decline appears, a tree may have been under drought stress for months. Contact Ross Tree now to evaluate watering needs before permanent damage occurs. To set an appointment, call (303) 871-9121 or click here to complete a request service form.
Frequently Asked Questions
Yes. Trees, shrubs, and perennial gardens can generally be watered with a hose, drip irrigation, or other approved methods outside restricted watering hours. Always check current Denver Water guidelines for specific restrictions.
Most newly planted trees should be watered two to three times per week during hot summer weather. The soil around the root ball should remain consistently moist but not saturated.
Using CSU’s rule of thumb: 10 gallons per inch of trunk diameter. The goal is to thoroughly soak the root zone rather than water frequently.
Leaf wilting, leaf scorch, premature yellowing, and early leaf drop are often the first visible signs that a tree is not receiving adequate moisture.
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