Yes — Strong Match
ageratum is grown as an annual, so season length decides it here, not a winter zone — and Colorado's typical season runs ~155 frost-free days against the ~60 ageratum needs.
Sow by about Jul 30
Ageratum is grown as an annual, so what decides it here is season length, not a winter zone. It needs about 60 frost-free days and a typical Colorado season runs ~155 frost-free days, leaving 95 days of slack. That is room to sow as late as about Jul 30 and still finish before the usual first freeze (Oct 8).
Want more weeks at either end of that window? How to buy the weeks →
60 frost-free days required against Colorado’s ~155-day season. NOAA Regional Climate Centers ACIS 1991-2020 (28°F killing freeze), state median across counties, via stateClimateData.ts.
Your yard isn't the whole zone.
Ageratum is grown as an annual, so your winter zone isn't the deciding factor — your frost-free window is. A typical Colorado site runs ~155 frost-free days, but that is a state median, not your yard: slope, trees, and low spots move the last-frost date across a single parcel. Enter your address and we'll score ageratum against your land's actual frost dates, sun, and soil.
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Zone Comparison
Ageratum Needs
- USDA Zones: Data pending
- Frost-Free Days: 60+
- Sun: Full Sun
- Mature size: 1.5 ft tall
- Picking window: ~90 days of harvest
- Drought tolerance: Low
Colorado Has
- USDA Zones: 3b-7b
- Last Hard Freeze (28°F): Apr 25 - May 19
- First Hard Freeze (28°F): Sep 29 - Oct 16
- Annual Rainfall: 7-20 inches
- Common Soils: Sandy loam, Clay loam, Alkaline caliche
Preferred Soil pH
Plant data: USDA PLANTS Database / Growable Ground plant catalog. State data: USDA ARS PHZM 2023, NOAA Climate Normals, NRCS SSURGO.
When to Plant Ageratum in Colorado
The frost window
Across Colorado, the last spring hard freeze (28°F) clears between Apr 25 and May 19, and the first fall hard freeze lands between Sep 29 and Oct 16 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 133-day window you can count on — up to 174 days on a mild site in a kind year. That marks the hard freeze, not the last light frost, so tender transplants usually wait another 2-3 weeks.
When to start the seed
Those dates are when ageratum goes out, not when it starts. It is sown under cover about 8 weeks ahead, so counting back from Colorado's late-side last freeze of May 19 is what fixes your sowing date — and a yard that clears frost earlier than the state's late date starts earlier still.
Days to maturity vs. the window
At 70 days to maturity (USDA PLANTS Database), counting from the last hard freeze it ripens with 63 days to spare even in Colorado's tightest freeze scenario — room for a later start or a second sowing.
Timing tuned to sub-state frost dates — Chaffee County, not the statewide average.
Frost window: NOAA Climate Normals 1991-2020. Plant timing fields: USDA PLANTS Database. Your site's own frost dates can run earlier or later than the state range — a parcel report pins them down.
Growing Season Fit
Frost-free days
Ageratum wants 60+ frost-free days; a typical Colorado site sees ~155 (NOAA Regional Climate Centers ACIS, 1991-2020). That leaves comfortable headroom for succession planting.
Growing degree days
Ageratum needs ~800 GDD (base 50°F) to ripen. The state median runs ~2083 GDD (from PRISM 1991-2020 climate normals), so Colorado's typical season clears that easily.
GDD and chill-hour figures are computed from PRISM 1991-2020 monthly climate normals; frost-free days from NOAA Regional Climate Centers ACIS, 1991-2020. Each is the state median across its counties (P25/P75 span the within-state spread). Hardiness-zone range is USDA PHZM 2023.
Soil + Drainage Fit
Ageratum likes near-neutral soil (pH 5.5-7). That's the common-ground band across Colorado's sandy loam and clay loam — a soil test confirms it for your site. It will grow across that whole span, but it does best between pH 6 and 6.5 — worth knowing before you decide whether a Colorado soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Colorado site is heavier clay or sits in a low spot, raised beds or amendment with compost solve it.
Your land, not the state average
Colorado's soils run mostly loam, but SSURGO maps the series, texture, and drainage under your exact parcel. What matters for ageratum is the horizon its roots actually work, not the surface. That map unit, not the state average, decides how ageratum performs.
Check your parcel → Source: USDA NRCS SSURGO.
Plant pH and drainage requirements from USDA PLANTS Database. Colorado soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
Ageratum in Colorado — Quick Answer
- Verdict: Yes — Strong Match
- Plant Zones: Data pending (USDA PLANTS Database)
- State Zones: 3b-7b (USDA ARS PHZM 2023)
- Growing Season: 28°F hard freezes about Apr 25 - May 19 to Sep 29 - Oct 16 (NOAA Regional Climate Centers ACIS, 1991-2020)
- Days to Maturity: 70 days
What Else to Consider for Ageratum in Colorado
Zone compatibility tells you about winter cold survival — but Colorado growers also need to think about:
Low annual rainfall (7-20 inches) means irrigation is essential nearly everywhere
High altitude UV and temperature swings stress plants
Very short growing season at elevation (60-90 frost-free days above 8,000 ft)
Alkaline soils (pH 7.5-8.5) limit acid-loving plants without amendment
Growing ageratum here specifically
Colorado's soils run mostly loam (Mollisols), and whether that suits ageratum's pH 5.5–7.0 preference comes down to your exact parcel, not the statewide picture.
Pull your parcel's SSURGO map unit, test pH, and amend toward ageratum's 5.5–7.0 target before planting. How to handle it →
What to actually do about these — and the constraints that apply to every Colorado grower, not just ageratum — is covered on the Colorado growing guide.
Timing shifts within Colorado
Colorado isn't one climate. In Chaffee County, the last hard freeze (28°F) holds until about Jun 12 — roughly 38 days later than the recorded state median — so plant ageratum to your county's window, not the statewide date.
Ageratum needs about 60 days of frost-free season to complete its cycle. Even Chaffee County — Colorado’s shortest season at 85 frost-free days — clears that by 25 days, so season length isn’t the constraint anywhere in Colorado. What varies is your own parcel.
Across Colorado’s 64 counties the frost-free season runs from 85 days in Chaffee County to 209 days in Mesa County — a spread of 124 days, which is why a single statewide planting date misleads. The median Colorado county sees 155 frost-free days, leaving ageratum about 95 days of slack on a typical parcel — room for a second sowing where the season runs long.
County last-freeze dates and frost-free day counts: NOAA Regional Climate Centers ACIS (1991-2020), county 28°F freeze (earlier than the folk 32°F "last frost"). Ageratum's 60-day requirement: USDA PLANTS / Growable Ground plant database (minimum frost-free days). A parcel report resolves your address's own frost dates.
Pollinator + Wildlife Value
Ageratum draws honeybees, native bees, and butterflies (moderate value, Growable Ground Plant Database). Planting it near vegetable beds can lift fruit set on neighboring crops. Outdoors, ageratum can only work that bloom inside Colorado's frost window — roughly 155 days, opening after the last 28°F freeze (Apr 25 - May 19) and closing at the first (Sep 29 - Oct 16), per NOAA Regional Climate Centers ACIS, 1991-2020.
Recommended Ageratum Varieties for Colorado
Colorado publishes no state variety trial for ageratum, so we won't invent a "best for Colorado" list. Choose types rated to your USDA hardiness zone (3b-7b), and confirm winter survival and drainage against your own parcel.
Colorado Cooperative Extension
For Colorado-specific cultivar recommendations, planting calendars, and pest pressure for ageratum, the canonical source is Colorado State University Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Ageratum native to Colorado?
No — the USDA PLANTS Database lists Ageratum as introduced rather than native in the Lower 48, so it is not part of Colorado's native flora. It grows here as a garden plant; pairing it with a few Colorado natives keeps local pollinators fed too.
Looking for plants that belong here? The Colorado growing guide lists USDA-documented natives for the state.
Native-range data: USDA PLANTS Database state-distribution records, accessed 2026-07-01.
Common Questions About Growing Ageratum in Colorado
When can I plant Ageratum in Colorado?
Colorado's last spring hard freeze (28°F) clears between Apr 25 and May 19, and the first fall hard freeze lands between Sep 29 and Oct 16 (NOAA Regional Climate Centers ACIS, 1991-2020). Time outdoor planting to after the last hard freeze for your specific site — a further two to three weeks for anything tender — and count back from those dates for transplant scheduling.
Can Ageratum mature before first frost in Colorado?
Yes — Ageratum matures in 70 days (USDA PLANTS Database), and Colorado's dependable hard-freeze-free window runs 133 days (NOAA Regional Climate Centers ACIS, 1991-2020), leaving 63 days of margin. Plant once the last hard freeze has passed — two to three weeks later for anything tender — and it ripens ahead of the first fall freeze.
What hardiness zone is Ageratum grown in across Colorado?
Colorado spans USDA hardiness zones 3b-7b (USDA ARS PHZM 2023). Ageratum carries a range of zones Data pending, so the overlap zones are where outdoor growing is most reliable.
How many frost-free days does a typical Colorado site have?
A typical Colorado site sees ~155 frost-free days per year (derived from NOAA Regional Climate Centers ACIS, 1991-2020). Ageratum needs 60+ frost-free days, so check whether your local microclimate runs above or below the state average before settling on a planting date. In cooler counties like Chaffee, the freeze-free season runs shorter than the state average, so verify your own county's window.
Is Ageratum native to Colorado?
No — the USDA PLANTS Database lists Ageratum as introduced rather than native in the Lower 48, so it is not part of Colorado's native flora. It grows here as a garden plant; pairing it with a few Colorado natives keeps local pollinators fed too.
How should I amend the soil for Ageratum in Colorado?
Ageratum prefers pH 5.5-7 and well (dry spells) drainage (Growable Ground Plant Database). That sits in the common-ground band across Colorado soils — a 30-minute soil test from a local Extension lab confirms it for your specific site.
Will Ageratum actually grow on my specific land in Colorado?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores ageratum against your address's exact soil pH, drainage, sun, and frost-date data drawn from USDA SSURGO, NOAA, and PRISM — not state averages.
Check your specific parcel in Colorado
State-level data is a sketch. Your Growable Ground report scores ageratum against your parcel's exact soil, sun, drainage, and frost data — not zone averages.
Three things about your lot that Colorado ageratum averages can't show:
We read public map data around Colorado — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Colorado ageratum average
Analysis by the Growable Ground research team, grounded in USDA PLANTS, USDA NRCS SSURGO, NOAA Climate Normals (1991-2020), and named Cooperative Extension sources. How we know →
Cite this dataset
Kirchick, M. (2026). Growable Ground Plant Database [Data set]. Zenodo. https://doi.org/10.5281/zenodo.21435643
Licensed CC BY-NC 4.0. DOI 10.5281/zenodo.21435643
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