Emerald ash borer in Colorado: treat or remove your ash, and what to plant instead

Treat or remove? How to decide

There is no quarantine to tell you what to do (Colorado repealed the EAB quarantine in 2019), so it comes down to the tree. EAB is now confirmed in Denver, Boulder, Fort Collins, and more than 20 Front Range cities, and it kills untreated ash, so doing nothing is really a slow decision to remove. Not sure the dieback is EAB and not something else? Why is my tree dying in Colorado helps you rule out the look-alikes first.

Treating makes sense when

• The tree is healthy, well placed, and one you would miss (good shade, good structure, less than about half its canopy thinned).
• You are within striking distance of a known infestation. Treatment is preventive and works best before the borer arrives or early in an infestation, not on a tree that is already collapsing.
• You accept that it is ongoing. Trunk-injection treatments are repeated every year or two, for the life of the tree, by a licensed applicator. It is a commitment, not a one-time fix.

Removing makes sense when

• The ash is already in decline (more than about half the canopy dead or thinning), poorly placed, or you do not want a recurring treatment bill.
• A dead ash gets brittle fast and becomes a hazard, so removing a declining one sooner is cheaper and safer than waiting.
• The lower-risk window to remove an ash is roughly September through April, when the borer is under the bark and not flying, which limits spread. Do not move ash wood or firewood off your property.

Either way, get a certified arborist to look at the specific tree before you commit. The numbers above are rules of thumb, not a diagnosis.

What to plant instead of an ash

A good ash replacement is tough in alkaline clay, lives a long time, and, just as important, is not the same species as every other tree on your block. Mix it up: the reason EAB hits so hard is that we planted ash everywhere. Here are eight proven, non-ash options, each a different genus.

• Oak, Bur Quercus macrocarpaThe bulletproof Front Range shade tree. Bur oak shrugs off our alkaline clay, drought, and wind, and lives for generations. It is slow, so buy the biggest one you can and be patient.
• Oak, Swamp White Quercus bicolorOak shade without the wait. Swamp white oak grows faster than most oaks, tolerates our clay, and colors up in fall, a strong like-for-like ash replacement for a lawn or parkway.
• Coffeetree, Kentucky Coffeetree Gymnocladus dioicusTough, clean, and underused. Kentucky coffeetree casts the same light, filtered shade ash were loved for, handles alkaline clay, and has no pest anywhere near EAB's league.
• Hackberry, Common Hackberry, PRAIRIE SENTINEL® Celtis occidentalisThe native survivor. Hackberry thrives on neglect in heavy clay and exposed sites where fussier trees sulk, and feeds birds in the bargain.
• Linden, American- American Sentry™, 'Redmond' Tilia americanaFast, dense, classic shade. American linden fills in quickly for a big summer canopy, with fragrant early-summer bloom that pollinators love.
• Catalpa, Northern/Western, HEARTLAND® Catalpa speciosaA fast, bold-leafed shade tree for a big spot. Northern catalpa is tough and quick, with showy white spring flowers, give it room.
• Oak, Chinkapin Quercus muehlenbergiiAn oak that actually prefers our dirt. Chinkapin oak is at home in high-pH limestone clay, so it dodges the iron chlorosis that yellows pin oak here.
• Honeylocust, Thornless Common – IMPERIAL®, SHADEMASTER®, SKYLINE® Gleditsia triacanthosFast filtered shade, with one caveat: honeylocust is already overplanted on the Front Range and is starting to show cankers. A fine tree, but if your block already has five, plant something else and keep the street diverse.