THE ROOT

The Amazing World of Galls: What They Are and What They Mean for Your

Trees

By Chad Szpunar

If you’ve ever noticed strange bumps, balls, or growths on your tree leaves or branches, you’ve likely

encountered one of nature’s most fascinating phenomena:

Galls.

While they can look alarming, most galls are harmless and highly specialized interactions between trees and

insects or mites. Understanding them is key to knowing when to act—and when to leave them alone.

What Are Galls?

Galls are abnormal plant growth that form when an insect, mite, or other organism manipulates plant tissue.

As the pest feeds or lays eggs, it releases chemicals that cause the tree to:

• Redirect nutrients
• Form protective structures
• Create a custom “home” for the organism

Each gall is species-specific, meaning different insects create distinct shapes, sizes, and locations on the tree.

What Causes Galls?

Most galls in Colorado are caused by:

• Tiny mites (often invisible to the naked eye)
• Small insects like wasps, aphids, or midges

These organisms trigger gall formation early in the growing season—often before you even notice them.

Common Types of Galls in Colorado

Leaf Galls

• Appear as bumps, blisters, or pouch-like structures
• Common on maples, oaks, and hackberries
• Often green, red, or yellow

Twig & Branch Galls

• Woody, round, or irregular growths on stems
• Can persist for multiple years

Bud & Flower Galls

• Distort normal growth
• Can impact appearance more than function

Are Galls Harmful to Trees?

In most cases: No.

Galls are primarily:

• Cosmetic
• Temporary
• Not a serious threat to overall tree health

Healthy, established trees can tolerate galls with little to no long-term impact.

When Should You Be Concerned?

While most galls are harmless, there are exceptions.

You may want a professional evaluation if you see:

• Heavy infestations across the entire canopy
• Repeated defoliation or stress year after year
• Significant branch dieback
• Galls combined with other stress factors (drought, poor soil, etc.)

In these cases, galls may be contributing to overall tree decline, especially in already stressed trees.

Why Galls Are Actually Fascinating

From an arborist’s perspective, galls are a remarkable example of:

• Plant-insect coevolution
• Highly specialized biological interactions
• Natural ecosystem processes

Each gall is essentially a custom-built structure, designed by the insect but grown by the tree.

Can Galls Be Treated?

Here’s the key point most homeowners don’t hear:

Once you see a gall, it’s already too late to treat it that season.

The triggering activity happens early—often before leaves fully emerge.

Management options include:

Monitoring

• Most effective approach for minor infestations

Cultural care

• Improve tree vigor through watering, soil health, and mulching

Preventative treatments (in select cases)

• Timed applications before gall formation
• Typically, only recommended for severe or recurring issues

The Bigger Picture: Tree Health Matters Most

Galls tend to be more noticeable—and sometimes more abundant—on trees that are:

• Stressed
• Growing in poor soil
• Lacking proper care

The best defense against any pest issue is a healthy, well-maintained tree.

HOA & Property Manager Insight

For HOAs, galls often generate concern because of their appearance.

However:

• Most do not justify chemical treatment
• Overreacting can lead to unnecessary costs
• Education is key to managing expectations

A professional tree care provider can help determine when action is warranted—and when it’s not.

Final Thoughts from an Arborist

Galls may look strange—even alarming—but in most cases, they are simply part of the natural relationship

between trees and their environment.

Understanding them helps you:

• Avoid unnecessary treatments
• Focus on overall tree health
• Make informed decisions about your landscape