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A pair of certified arborists tend to an ash tree in Denver's City Park | Be A Smart Ash Denver

Emerald ash borer treatment options for your Denver ash tree

DENVER — The emerald ash borer (EAB), which feeds on ash trees and is now the most destructive forest pest in U.S. history, has arrived in Colorado, and it’s knocking on Denver’s door. That fact is overwhelming in its own right.

But you might find yourself even more overwhelmed by all the treatments options available to protect your ash tree from EAB. That’s where we come in!

For starters, there are four treatment options that have been examined at length as part of a multi-state insecticide study from Colorado State, Ohio State, Michigan State and Purdue universities. Below is a breakdown of each option.

You might also be happy to know that your Friendly Denver City Forester has put together a list of Certified Smart Ash tree professionals, all of whom have the right mix of know-how and tools to safely and effectively protect your tree from EAB.

  1. Trunk injection
    • Who applies treatment: Licensed pesticide applicators
    • Treatment process: A licensed pesticide applicator will drill through the bark and into the outer sapwood of the ash tree, where they will inject the insecticide
    • Frequency: 1-3 years, depending on selected product
    • Effectiveness: When properly administered by a licensed professional, some trunk injections have shown to be 90 percent effective at controlling EAB, making this treatment the most effective method by far
    • Drawbacks: Cost and potential drilling wounds. While the most effective, trunk injections that last multiple years are typically more expensive than other EAB treatments.
  2. Soil drench
    • Who applies treatment: Licensed pesticide applicators, or property owners (if ash tree is under 15” diameter at 4.5’ off the ground)
    • Treatment process: Though soil drenches vary, they’re typically applied in the form of liquid or as granules that are then watered into the soil around the tree after the area beneath the tree has been cleared, thus allowing the soil drench to take hold in the roots of the tree
    • Frequency: The ideal frequency varies by product and the size of the ash tree being treated. But soil drench treatments should not be applied more than once a year.
    • Effectiveness: Inconsistent. In some trials, EAB control was excellent, while others yielded poor results. That said, soil drenches administered by tree professionals proved to be more effective.
    • Drawbacks: Effectiveness and environmental impact. While they proved to be better EAB control than no treatment, soil drenches are significantly more likely to fail than trunk injections. They’re also more likely to impact plants, insects or animals that may be near the tree.
  3. Bark spray
    • Who applies treatment: Licensed pesticide applicators
    • Treatment process: This formulated insecticide is sprayed on the bottom five to six feet of the ash tree using a common garden sprayer. That insecticide then penetrates the bark and is transported systemically throughout the tree.
    • Frequency: Once a year
    • Effectiveness: Inconsistent. The results of effectiveness testing on bark sprays have yielded similar results when compared to the testing on soil drenches.
    • Drawbacks: Effectiveness and environmental impact. Pesticide can blow onto adjacent plants, potentially impacting plants, insects or animals nearby.
  4. Tree removal and replacement
    • Who applies treatment: Professionals
    • Treatment process: While some property owners may be savvy enough to plant a tree on their own, removing a tree is a complicated process that requires a tree professional – often times for permitting reasons
    • Frequency: Just once
    • Effectiveness: 100 percent. While getting a new tree to take hold presents its own challenges, removing and properly disposing of an ash tree is 100 percent effective at controlling the spread of EAB.
    • Drawbacks: Losing a tree that may have intrinsic value to the property owner

So which EAB treatment method is the best for ash trees in Metro Denver, an area that includes 1.45 million ash trees? Unfortunately, there isn’t one right answer.

The best treatment for your Denver ash depends on a variety of factors, and it really comes down to property owner preference, according to Denver City Forester Rob Davis.

“Step one has to be educating yourself — gain an understanding of the options and then make a decision about what’s best for you,” Davis said. “In reality, the best place to start is with a good arborist.”

So what defines a good arborist? Start with one who has been licensed by the Colorado Department of Agriculture to apply pesticides.

Note that fall and winter are not recommended times to treat or remove your ash tree. It is, however, a great time to make an appointment to treat or remove your ash tree, as many reputable tree care companies book out months in advance. And if you plan on trying to save your ash, make sure to water it occasionally throughout the winter to keep it healthy.

Lastly, note that while it’s true that EAB is yet to be discovered in Denver, the pest has already been found in Boulder and Longmont. And that means it’s just a matter of time before it arrives in Denver. What’s more, if you wait to map out your EAB defense plan until the pest has been discovered, it may well be too late.

So don’t delay: Begin making your EAB defense plan today!