Are Front Range cities still planting ash trees?

Barely. In Denver, ash is only 1.9% of the youngest trees (under 6-inch trunks) versus 20.1% of the mature canopy, and Denver and Aurora now prohibit new ash plantings. The shift away from ash is clear in both policy and the recently-planted trees.

Is the Front Range's tree problem fixed?

Not yet. Cities have largely stopped planting ash and banned the worst monoculture cultivars, but honeylocust is still heavily recommended, Colorado nurseries still sold thousands of ash in 2024, and a truly diverse canopy takes decades to grow in. The trend is good; the job is not done.

What are cities planting instead?

A deliberately rotated mix. Giveaway programs spread plantings across dozens of species each year, and city lists steer toward oaks, hackberry, Kentucky coffeetree, and other underused trees, precisely to avoid the next single-pest wipeout.

Is the Front Range fixing its trees?

Compiled by Nicholas Julia · Reviewed against the 2024 Front Range Tree Recommendation List, CSU Extension & Plant Select® · Updated 2026-06-10

Part 2 of Front Range Planting by the Numbers. Part 1 measured the gap between what we plant and what survives here: What we plant vs what survives →

Quick answer

Mostly yes, and you can measure it. In Denver, ash is just 1.9% of the youngest trees (recently planted) versus 20.1% of the mature canopy, and Denver and Aurora now ban new ash plantings. Cities have clearly turned away from the ash monoculture, though nurseries still sell ash and honeylocust is still over-recommended, so the job is not finished.

The youngest trees tell the story

No inventory records when a tree was planted, but Denver and Boulder both record trunk diameter, a fair proxy for age: the smallest trees went in most recently. So the share of ash among the youngest trees versus the mature canopy is a direct read on whether cities are still planting it.

They are not. In Denver, ash is just 1.9% of the youngest trees (0 to 6 inch trunks) but 20.1% of the mature cohort that defined the old canopy. That is the difference between about 1 in 50 and 1 in 5. Boulder shows the same turn. (The very oldest trees carry less ash too: they predate the mid-century boom that filled the mature cohort with it.)

Trunk size (age proxy)	Denver ash	Boulder ash
0-6 in (youngest)	1.9%	2.7%
6-12 in	14.9%	5.4%
12-24 in (mature)	20.1%	13.5%
24+ in (oldest)	8.3%	7.5%
Ash (Fraxinus) share by trunk diameter, from the Denver and Boulder public tree inventories. Diameter is a population-level age proxy, not exact age.

Cities banned what they over-planted

The planting data is backed by policy. Denver's approved street-tree rules put ash (Fraxinus), silver maple, Autumn Blaze / Freeman maple, and Sunburst honeylocust under a planting moratorium - the city no longer allows them in new public plantings. [City and County of Denver] Aurora, where emerald ash borer is established, is removing and replacing its city-owned ash and no longer plants new ash, steering toward a more diverse mix. [City of Aurora Forestry]

That is the loop closing: the species the inventories show were over-planted, and that the 2024 Front Range list rates Not Recommended, are the same ones cities now refuse to plant. The policy caught up to the problem.

Treating, not just losing

Replacing ash is only half of it. Longmont is the one inventory that publishes whether each ash is being treated against emerald ash borer, and it is treating most of them: 1,420 of 1,882 tracked ash (75.5%) are under treatment, with only 455 left untreated. Cities are fighting for the mature canopy while they diversify the young one.

But not all the way

The turn is real, not complete. Colorado nurseries were still selling thousands of ash shade trees in 2024, so ash has not left the supply chain. [USDA NASS] Honeylocust, already one of the most-planted trees on the Front Range, is still on city giveaway menus and recommended lists, and a few cities' approved lists have not been updated in over a decade. Diversifying away from ash is not the same as planting a genuinely varied canopy yet.

What gets handed out now

The programs that put trees in residents' yards have leaned into variety. Fort Collins's annual resident tree sale offers about 1,000 trees across 32 species for $25 each, up from 200 trees in 2018. [City of Fort Collins Forestry (tree canopy / resident tree sale)] Denver Digs Trees, the city's flagship distribution program, hands out roughly 1,500 trees a year across about 11 species, and the mix changes from year to year. [The Park People] The rotation is the point: spread the bets so the next pest cannot take out a fifth of the canopy at once.

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Methodology and sources

The diversification numbers are computed from the same public city tree inventories as Part 1: we group each city's trees by trunk diameter and measure the ash share in each band. Diameter is an age proxy, not exact age, so read the bands as a population trend, not a birthday. The treatment figure is Longmont's own data. The policy, supply, and giveaway figures are third-party and cited inline.

Inventories

Denver: City and County of Denver Tree Inventory (Parks & Recreation), data.colorado.gov/wz8h-dap6
Boulder: City of Boulder Tree Inventory (Open Data)
Longmont: City of Longmont Public Tree Inventory (Forestry), TREATED field

Cited figures

Denver's approved street-tree rules put ash (Fraxinus), silver maple, Autumn Blaze / Freeman maple, and Sunburst honeylocust under a planting moratorium - the city no longer allows them in new public plantings. City and County of Denver, Office of the City Forester, Approved Street Tree List (accessed 2026-06-23).
Aurora, where emerald ash borer is established, is removing and replacing its city-owned ash and no longer plants new ash, steering toward a more diverse mix. City of Aurora Forestry, Emerald Ash Borer (accessed 2026-06-23).
Colorado nurseries were still selling thousands of ash shade trees in 2024, so ash has not left the supply chain. USDA NASS, 2024 Census of Horticultural Specialties (Colorado) (accessed 2026-06-23).
Fort Collins's annual resident tree sale offers about 1,000 trees across 32 species for $25 each, up from 200 trees in 2018. City of Fort Collins Forestry (tree canopy / resident tree sale) (accessed 2026-06-23).
Denver Digs Trees, the city's flagship distribution program, hands out roughly 1,500 trees a year across about 11 species, and the mix changes from year to year. The Park People, Denver Digs Trees (accessed 2026-06-23).

Frequently asked questions

Are Front Range cities still planting ash trees?: Barely. In Denver, ash is only 1.9% of the youngest trees (under 6-inch trunks) versus 20.1% of the mature canopy, and Denver and Aurora now prohibit new ash plantings. The shift away from ash is clear in both policy and the recently-planted trees.
Is the Front Range's tree problem fixed?: Not yet. Cities have largely stopped planting ash and banned the worst monoculture cultivars, but honeylocust is still heavily recommended, Colorado nurseries still sold thousands of ash in 2024, and a truly diverse canopy takes decades to grow in. The trend is good; the job is not done.
What are cities planting instead?: A deliberately rotated mix. Giveaway programs spread plantings across dozens of species each year, and city lists steer toward oaks, hackberry, Kentucky coffeetree, and other underused trees, precisely to avoid the next single-pest wipeout.

← Back to Part 1: What we plant vs what survives here