Protecting Your Ash Trees in Colorado

As a Colorado arborist, I can tell you this with absolute confidence: If you have ash trees, it needs protection.

Ash trees make up a huge portion of Colorado’s urban canopy. They provide shade, cooling, curb appeal, and significant property value. But they’re also among the most vulnerable species in our landscape—targeted by multiple destructive pests.

Most homeowners have heard of Emerald Ash Borer (EAB), but what many don’t realize is that ash trees in Colorado face a whole lineup of borers, each capable of serious damage if left unaddressed. Protecting these trees is not optional—it’s essential for their survival.

Here’s what every Colorado homeowner should know.

Why Ash Trees Are at High Risk in Colorado

Colorado’s climate puts ash trees under several stresses:

• Heat and drought
• High altitude and intense sunlight
• Compact, alkaline soils
• Irrigation challenges
• Winter desiccation

Stressed trees are easier targets for borers, especially the native beetle species. Even before EAB arrived in Colorado, ash trees were already at risk from borers—EAB just made the threat more urgent and severe.

The Major Threats to Ash Trees in Colorado

1. Emerald Ash Borer (EAB) – The Most Destructive

EAB is the most serious insect threat we’ve ever seen for ash trees. Unlike native borers, EAB doesn’t wait for a tree to be stressed or declining.
It attacks healthy, vigorous ash trees and kills them within 2–4 years without treatment.

Signs include:

• Thinning canopy
• D-shaped exit holes
• Increased woodpecker activity
• Splitting bark
• Epicormic (stress) shoots

Once symptoms appear, it’s usually too late to save the tree. Preventive treatment is the only effective strategy.

2. Lilac Ash Borer – A Common, Native Pest

The Lilac Ash Borer is attracted to ash trees that are stressed from:

• Drought
• Overpruning
• Soil compaction
• Winter injury

This borer tunnels into the trunk and larger branches, causing:

• Bark splitting
• Limb dieback
• Reduced vigor
• Increased susceptibility to breakage

While rarely fatal by itself, it severely weakens trees.

3. Banded Ash Borer – Early-Season Invader

This beetle often attacks weakened ash trees, especially during the spring. It prefers trees already stressed by:

• Winter drought
• Poor soil conditions
• Mechanical injury

Damage includes:

• Tunneling beneath bark
• Disrupted nutrient flow
• Progressive canopy thinning

4. Redheaded Ash Borer – A Fast Worker

This borer is widespread in Colorado and known for its quick life cycle. It commonly targets recently transplanted or newly stressed ash trees.

Signs of attack:

• Frass (sawdust-like material)
• Small round exit holes
• Rapid canopy decline in young trees

5. Ash Bark Beetle – Tiny, But Damaging

Small but numerous, the ash bark beetle can cause:

• Branch dieback
• Bark cracking
• Overall canopy thinning

They exploit weakened or drought-stressed trees and can cause dieback across the entire upper canopy if not managed.

Why Preventive Treatment Matters

As an arborist, I emphasize prevention because borer infestations are usually invisible until the damage is advanced.

Here’s why treatment is essential:

1. EAB Will Kill Unprotected Ash Trees

None of Colorado’s ash trees have natural immunity to EAB. Without treatment, mortality rates approach 100%.

2. Other Borers Exploit Stress

Lilac ash borer, redheaded ash borer, ash bark beetle, and banded ash borer all target weakened trees.
Even if EAB isn’t present in your immediate area, these pests can cause significant decline.

3. Treatment Is Far Cheaper Than Removal

The average ash removal in Colorado:

• $1,500–$6,000 for a mature tree
• More if the tree is close to structures or utilities

Preventive treatments cost a fraction of that and protect your landscape and property value.

4. Healthy Ash Trees Add Value

A large, mature ash tree can be worth thousands of dollars in:

• Cooling savings
• Shade
• Aesthetics
• Increased property value

Keeping them healthy is an investment.

What Effective Protection Looks Like

1. Systemic Injections (Most Effective for EAB)

Every 1–2 years, an arborist applies a systemic treatment directly into the trunk.
These treatments:

• Prevent EAB infestations
• Kill active borers
• Boost tree resilience

This is the gold standard for EAB protection.

2. Preventive Bark Sprays

Effective for:

• Lilac ash borer
• Redheaded ash borer
• Ash bark beetle

These sprays protect vulnerable bark surfaces and discourage egg-laying.

3. Watering and Fertilization

Healthy trees are less attractive to native borers.
An arborist may recommend:

• Winter watering
• Deep-root fertilization
• Mulching

These steps reduce stress, making the tree more resistant to attack.

4. Regular Monitoring

Annual inspections help catch early signs of:

• Borer activity
• Stress
• Decline
• Structural issues

Early detection can save a tree.

The Arborist’s Bottom Line

If you have an ash tree in Colorado, protecting it is not optional—it’s urgent.

Between native borers and the reality of Emerald Ash Borer spread across the state, the only way to save an ash tree long-term is with consistent, preventive care.

Homeowners have two choices:

1. Treat and Preserve the Tree

(safest, most cost-effective, environmentally responsible)

2. Plan for Removal and Replacement

(often far more expensive and disruptive)

As an arborist, my recommendation is simple:
If your ash tree is healthy enough to save, protect it now—before pests take hold.
Once symptoms show, the battle is usually lost.