Did the Front Range Plant the Wrong Trees?

What we plant vs what survives here

A good chunk of what Front Range cities plant is exactly what the 2024 list now says to stop planting. To measure it, we pulled the public, city-managed tree inventories for Denver, Aurora, Boulder, and Longmont (street right-of-way plus park and other public-property trees), counted what is actually in the ground, and checked each species against the 2024 Front Range Tree Recommendation List.

Ash is the clearest case. About 15% of the trees in Colorado's urban forests are ash, the species emerald ash borer kills. [Colorado State Forest Service] Emerald ash borer is now established here, so untreated ash is on borrowed time. Our own count of Denver's public trees puts green and white ash at 11.1% of the total, and the city agrees: Denver's own 2024 Urban Forest Strategic Plan reports that of the city's 117,496 public trees, ash and maple each make up over 10% of the canopy (about 14% ash, 12% maple). [City and County of Denver] The canopy leans on a short list either way: the five most-planted species are 27.2% of every public tree Denver tracks.

The 10-20-30 rule is the standard test for a healthy urban canopy: no single species should be more than 10% of it, and no single genus more than 20% [PlanIT Geo]. By that test, most of these cities are more mixed than their "ash monoculture" reputation, though Aurora leans heavily on honeylocust. The shared weak spot is narrower: ash, plus a handful of over-planted species the 2024 list now rates Not Recommended. Here is each city:

Denver: 319,190 public trees, 404 species

In Denver, all ash species together are 11.1% of the public trees. The single most-planted species is Green Ash (7.8%), the biggest genus is Acer (14.4%), and in all, 22.5% of the public trees are species the 2024 list now rates Not Recommended.

Aurora: 60,939 public trees, 250 species

In Aurora, all ash species together are 9.5% of the public trees. The single most-planted species is Honeylocust (16.3%), the biggest genus is Gleditsia (16.3%), and in all, 17.8% of the public trees are species the 2024 list now rates Not Recommended.

Boulder: 49,924 public trees, 436 species

In Boulder, all ash species together are 7.2% of the public trees. The single most-planted species is Honeylocust (7.2%), the biggest genus is Acer (12.5%), and in all, 14.7% of the public trees are species the 2024 list now rates Not Recommended.

Longmont: 22,061 public trees, 281 species

In Longmont, all ash species together are 8.2% of the public trees. The single most-planted species is Honeylocust (6.7%), the biggest genus is Pinus (11.6%), and in all, 11.3% of the public trees are species the 2024 list now rates Not Recommended.

The cities we cannot join directly tell the same story. Fort Collins does not publish a joinable species inventory, but its 2025 Urban Forest Strategic Plan reports green ash at about 12% and honeylocust about 11% of its roughly 60,000 public trees (both above the recommended 10% single-species limit), with emerald ash borer threatening the 13% of public trees that are ash. [City of Fort Collins]

The Front Range is not an outlier. A study of more than 5 million trees across 63 US cities found maples and oaks dominate urban canopies nationwide and green ash is among the most common species, with a median effective diversity of just 26 species per city (native share ranged widely, median 45.6%). [McCoy et al.] Green ash being one of the most-planted city trees in the country, just as emerald ash borer spreads, is a national problem the Front Range shares. The fix is the same everywhere: plant a wider mix.

Underused trees to plant instead

Plenty of good trees barely show up in the inventories. These are catalog trees rated Recommended or Recommended for Most Sites, one per genus, leaning native and pollinator-friendly, so you can spread the mix instead of adding another ash or maple.

• Hawthorn Crataegus douglasii Colorado native, high pollinator value
• Juniper Juniperus scopulorum Colorado native
• Douglas-fir Pseudotsuga menziesii var. glauca Colorado native
• Fir Abies concolor Colorado native
• Goldenrain tree Koelreuteria paniculata high pollinator value
• Yellowwood Cladrastis kentukea high pollinator value
• Chokecherry Prunus virginiana high pollinator value
• Dogwood Cornus mas high pollinator value

Are cities fixing this? Part 2 measures it: ash is a fraction of recently-planted trees, and Denver and Aurora now ban new ash. Is the Front Range fixing its trees? →

The mismatch, mapped

The gap is not evenly spread. Denver leans hardest on species the list now flags, and within Denver the ash exposure tracks the age of the neighborhood: downtown and the oldest core lean least on ash, while the postwar neighborhoods on the south and southwest edge lean hardest, where ash can top a quarter of the public trees.

Denver's ash, block by block

Ash share of street and park trees, 3% downtown to 27% in Goldsmith and Marston. The 5 highest-ash neighborhoods are labeled.

Tip: hover any neighborhood on the map for its name and exact ash share.

The catalog by the numbers

The whole rated catalog comes to 494 plants: 228 trees, 96 shrubs, 109 perennials, and 61 groundcovers.

On the 2024 tree list specifically, of 228 trees rated, 16 are Not Recommended outright, and the rest spread across Recommended, Recommended for Most Sites, and Conditionally Recommended. Low-water plants dominate the catalog because they are what survives our clay, alkalinity, and dry winters.

The Front Range Tree Toughness Index

We scored every recommended tree 0 to 10 for raw toughness: hail, wind, and salt tolerance, drought hardiness, and how long it lives. One per genus, so the list is varied, not five junipers. The top of the index shrugs off a Front Range summer; the bottom asks for more than it gives back.

And the most demanding

The other end of the same scale: thirsty, brittle, or short-lived enough to be a poor bet here.

Every city has a tree that loves it there

A handful of natives top almost every Front Range city: One: Seed Juniper (8 cities), Rocky Mountain Juniper (8 cities), Gambel Oak (4 cities), Arizona Cypress (3 cities), Bristlecone Pine (2 cities). They are the dependable backbone. More interesting is each city's most distinctively suited tree, the one that fits its particular mix of hail, wind, cold, fire risk, and deer better than it fits anywhere else.

Distinctiveness is the gap between a tree's hazard-fit score for that city and its average score across all nine, so it surfaces the local specialist rather than the universal all-star.

How the Front Range compares

Against the rest of the country, Denver and Aurora plant strikingly few state-native trees, 13.5% and 8.6%. The fairer test, controlling for climate, is against other dry Western cities, which work from a similarly thin native-tree palette: Albuquerque (25.5%) and Las Vegas (26.7%) still plant roughly two to three times the native share Denver and Aurora do, and Colorado Springs, leaning on native ponderosa pine, reaches 30.3%. So part of the Front Range gap is real constraint (Colorado has few native shade trees that thrive in town), but not all of it: it is also a planting habit.

Across the full 63-city study [McCoy et al.] the median is about 46% state-native, but that median is pulled up by cold-Midwest cities like Minneapolis and Milwaukee, whose native tree pool runs far deeper than Colorado's, so the climate-matched comparison above is the one that holds.

One thread ties it together: green ash is the single most-planted tree in Denver, and in Minneapolis and Milwaukee too. The Front Range shares its ash reliance with the cold-Midwest cities now losing those trees to emerald ash borer. Denver's overall diversity is fine (about 49 effective species); the exposure is specific: too much ash, too few natives.

What it costs, and what your city pays back

A Kentucky bluegrass lawn needs roughly 18 to 20 gallons of water per square foot per year, while native grasses use about 6, according to Denver Water figures cited by 5280. [5280] Denver Water's water-use benchmark for a typical mixed-use landscape is about 12 gallons per square foot per year. [Denver Water] That gap is why nearly every Front Range water provider now pays you to pull out thirsty turf. Ranked by the most comparable number, dollars per square foot of lawn removed (5 of the nine cities publish one):

Amounts are quoted verbatim from each provider and were verified against their conservation pages; some are flat caps, some are per-square-foot, and toilet or nozzle rebates are excluded.

Whatever kind of gardener you are

• Dog owners. 17.2% of the catalog (85 plants) is toxic to dogs, which means the other 82.8% is not. See the plants toxic to dogs list to know which is which.
• Pollinator gardeners. 53.8% (266 plants) rate high for pollinator value, concentrated in the perennials and shrubs.
• Native-plant advocates. 27.9% (138 plants) are Colorado or regional natives. See native trees to start.

That is the problem. Is the Front Range fixing it? The good news: the planting habits are already changing, and you can measure it. Ash is a fraction of recently-planted trees, and Denver and Aurora now ban new ash. Read Part 2: Is the Front Range fixing its trees? →

Methodology and sources

Two kinds of numbers here, kept separate on purpose. The internal figures (the catalog composition, the toughness scores, the per-city distinctiveness) are computed straight from our published 480-plant dataset and the committed inventory snapshots, so anyone can re-derive them. The external figures are third-party data we cannot recompute, and each is cited inline where it appears.

The catalog is rated against the 2024 Front Range Tree Recommendation List, CSU Extension, and Plant Select. Inventory species are matched to the catalog by botanical name, falling back to the genus where the list rates a whole genus (this is how all ash resolves to Not Recommended). Records identified only to genus (an inventory label like "maple species") join only at that ash-or-pear genus level, so they are not counted as Not Recommended even if some are; the Not-Recommended shares are therefore a conservative estimate, and a floor for those genus-level records. The data is public. Check our math.

Three caveats. First, the municipal inventories are open data and carry a little record-level noise. Denver and Boulder include empty planting sites and stumps, which we drop (Longmont publishes standing trees only); and across all three there is the occasional blank or mismatched common name. We aggregate by botanical name and show the most common label, so a stray mislabel cannot mis-rank a species; any residual sits well under a tenth of a percent. Second, our three internal cuts of the Denver inventory, the mismatch count here, the neighborhood map, and Part 2's trunk-diameter analysis, apply slightly different vacant-site and stump filters and were pulled on slightly different dates, so the Denver tree total ranges from about 319,000 to 362,000 and the overall ash share lands in a 10 to 11% band; we use 11% as the headline. Third, where we put our own ash count (about 11% of roughly 319,000 public trees) next to the city's (14% of 117,496), the two use different inventories, dates, and totals; we cite both because they agree on the point that matters, that ash sits above the 10% line.

What we plant: municipal inventories

• Denver: 319,190 living trees across 404 species, from City and County of Denver Tree Inventory (Parks & Recreation) (accessed 2026-06-22).
• Boulder: 49,924 living trees across 436 species, from City of Boulder Tree Inventory (Open Data) (accessed 2026-06-22).
• Aurora: 60,939 living trees across 250 species, from City of Aurora Public Tree Inventory (Open Data) (accessed 2026-06-22).
• Longmont: 22,061 living trees across 281 species, from City of Longmont Public Tree Inventory (Forestry) (accessed 2026-06-22). This inventory records species as common names; we resolved 99.8% of trees to botanical names using a crosswalk built from the Denver and Boulder data (the rest are left unclassified).

Cited external figures

• About 15% of the trees in Colorado's urban forests are ash, the species emerald ash borer kills. Colorado State Forest Service, 2025 Report on the Health of Colorado's Forests (Emerald Ash Borer) (accessed 2026-06-22).
• Denver's own 2024 Urban Forest Strategic Plan reports that of the city's 117,496 public trees, ash and maple each make up over 10% of the canopy (about 14% ash, 12% maple). City and County of Denver, Urban Forest Strategic Plan (2024) (accessed 2026-06-22).
• A study of more than 5 million trees across 63 US cities found maples and oaks dominate urban canopies nationwide and green ash is among the most common species, with a median effective diversity of just 26 species per city (native share ranged widely, median 45.6%). McCoy et al., "Species clustering, climate effects, and introduced species in 5 million city trees across 63 US cities" (eLife, 2022) (accessed 2026-06-22).
• The widely used 10-20-30 rule for urban forest diversity: no more than 10% of a tree population from one species, 20% from one genus, or 30% from one family. PlanIT Geo, "The 10-20-30 Rule for Tree Diversity" (accessed 2026-06-22).
• A Kentucky bluegrass lawn needs roughly 18 to 20 gallons of water per square foot per year, while native grasses use about 6, according to Denver Water figures cited by 5280. 5280, "A Beginner's Guide to Xeriscape in Denver" (citing Denver Water) (accessed 2026-06-22).
• Denver Water's water-use benchmark for a typical mixed-use landscape is about 12 gallons per square foot per year. Denver Water, "How much water does your landscape really need?" (accessed 2026-06-22).
• Fort Collins does not publish a joinable species inventory, but its 2025 Urban Forest Strategic Plan reports green ash at about 12% and honeylocust about 11% of its roughly 60,000 public trees (both above the recommended 10% single-species limit), with emerald ash borer threatening the 13% of public trees that are ash. City of Fort Collins, Urban Forest Strategic Plan (2025) (accessed 2026-06-22).

Download the data

These are the numbers behind this study, the per-city counts, shares, and ratings, in one file you can sort and check yourself. Download the data (CSV).

Charts you can run with a story (SVG, free to reuse with credit): most-planted by rating, ash by age.

Everything here is free to reuse under CC BY 4.0: credit Front Range Plant Finder and link back to this page. Reporters, see our press page for the full kit, or email press@coloradogardening.co for a PNG or a quick fact check.

Source: Front Range Plant Finder (coloradogardening.co), "Did the Front Range Plant the Wrong Trees?", CC BY 4.0.