Is the Front Range fixing its trees?
Part 2 of Did the Front Range Plant the Wrong Trees? Part 1 found that roughly 1 in 5 of Denver's public trees is a species the city's own 2024 list says to stop planting, with ash the biggest exposure. Here is the evidence that cities are correcting course. Read Part 1 →
Quick answer
Denver has nearly stopped planting ash: it is about 1 in 50 of the youngest street trees (1.9%) versus about 1 in 5 of the mid-life cohort (20.1%). So mostly yes, the Front Range is fixing it, and you can measure it: Denver and Aurora now ban new ash plantings. Cities have clearly turned away from ash, but nurseries still sell it and honeylocust is still over-recommended, so the job is not done yet.
What the youngest trees show
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 if cities have stopped planting ash, the youngest trees should carry far less of it than the older ones.
They do. In Denver, ash is just 1.9% of the youngest trees (0 to 6 inch trunks) but 20.1% of the mid-life cohort (12 to 24 inch trunks), the trees that went in when ash was a go-to street pick. That is the difference between about 1 in 50 and 1 in 5. Ash does not simply climb with age: the share peaks in that mid-life band and falls again in the oldest trees (8.3%), which fits a wave of ash planting decades ago rather than a steady default. Boulder shows the same pattern.
Cities are planting far less ash, and emerald ash borer (plus the removals it forces) is thinning the mature ash already standing. The planting bans below confirm the first one is real, not just a side effect of trees coming down.
Ash share of Denver trees by trunk size: the mid-life cohort versus the youngest, recently-planted trees.
| 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 (mid-life) | 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]
The policy matches the data: 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.
Treating, not just losing
Cities are not only replacing ash, they are trying to keep the big old trees alive. 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. They treat the mature ash they already have, and plant something other than ash underneath.
But not all the way
The shift away from ash is real, but it has limits. 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 still leaves the Front Range a long way from a genuinely varied canopy.
What gets handed out now
The programs that put trees in residents' yards now spread them across far more species. 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] Plant a wider mix, and the next pest cannot take out a fifth of the canopy the way emerald ash borer is taking out ash.
Plan your garden
Not sure what to plant? The finder matches Front Range trees, shrubs, perennials, and groundcover to your soil, water, sun, and zone.
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 mid-life cohort, and Denver and Aurora now prohibit new ash plantings. The shift away from ash shows 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 varied canopy takes decades to grow in.
- 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: 1 in 5 Front Range city trees are Not Recommended
Get the Front Range planting guide
A free monthly email: what to plant, what to avoid, and when, for our clay, hail, and dry winters. No spam, unsubscribe anytime.