Unlikely — Zone Mismatch
siver ironbark (zones 9-11) does not overlap with Colorado's zone range (3b-7b). Growing outdoors is unlikely without significant protection.
2 zones short — what would close it
Siver Ironbark is hardy to zone 9, and the warmest ground in Colorado is zone 7 — 2 zones short. Planted in open ground it will not survive a Colorado winter. To grow it here, grow siver ironbark under glass — a greenhouse is the only lift this wide the engine models.
A greenhouse is the lift this needs. How a greenhouse changes what you can grow →
Zone math from 9-11 (plant) against 3-7 (Colorado). Lift modeled by crop_engine Sprint 20B effective_zone_modifier (+2 zones) via api/fast_sections.py Unlock Engine v2.
Your yard isn't the whole zone.
Colorado spans zones 3b-7b, but your yard sits in exactly one — and slope, tree cover, and cold-air pockets nudge it further. Enter your address and we'll score siver ironbark against your parcel's actual hardiness, soil, and sun.
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Zone Comparison
Siver Ironbark Needs
- USDA Zones: 9-11
- Frost-Free Days: 150+
- Sun: Full Sun
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
Plant Zone Range (zones 9-11)
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 Siver Ironbark 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.
Winter standing
Colorado runs as cold as USDA zone 3, and no sourced winter-hardiness floor is on record for siver ironbark, so this page does not state whether it overwinters here — and gives no fall establishment date on the strength of a guess. Whether siver ironbark is a permanent planting or a replant-every-spring crop for you turns on your own zone within Colorado; check its hardiness range against that before treating it as long-lived ground.
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
The zone check says siver ironbark does not survive a Colorado winter in the ground. The season figures below measure a different question — whether it could finish a season here at all — and they only bear on Colorado under protection.
Frost-free days
Siver Ironbark wants 150+ frost-free days; a typical Colorado site sees ~155 (NOAA Regional Climate Centers ACIS, 1991-2020). That leaves a tight window — use transplants and pick early-maturing cultivars.
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
Siver Ironbark likes near-neutral soil (pH 5.5-7.5). 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 7 — 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. Siver Ironbark needs about 59 inches of workable depth, so it is that horizon — not the surface — you are testing for. That map unit, not the state average, decides how siver ironbark 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.
Siver Ironbark in Colorado — Quick Answer
- Verdict: Unlikely — Zone Mismatch
- Plant Zones: 9-11 (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)
What Else to Consider for Siver Ironbark 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 siver ironbark here specifically
Siver Ironbark wants pH 5.5–7.5 and rates to USDA zones 9–11, but Colorado's soils are dominantly loam — the fit is decided by your parcel's own map unit, not the state average.
Match siver ironbark to your parcel's SSURGO map unit — test pH and texture, and amend toward its 5.5–7.5 range. How to handle it →
What to actually do about these — and the constraints that apply to every Colorado grower, not just siver ironbark — 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 siver ironbark to your county's window, not the statewide date.
Siver Ironbark needs about 150 days of frost-free season to complete its cycle. 33 of Colorado’s 64 counties give it that outright. The tightest that does is Archuleta County at 150 frost-free days — 0 to spare. Below that line you are transplanting or extending the season, not direct-sowing.
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 siver ironbark about 5 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"). Siver Ironbark's 150-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
Siver Ironbark draws honeybees and native bees (high value, Growable Ground Plant Database). Planting it near vegetable beds can lift fruit set on neighboring crops. Outdoors, siver ironbark 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.
Good to Know Before You Plant Siver Ironbark
Siver Ironbark is listed as toxic to dogs, cats (leaves) at a moderate level (ASPCA). Most listed plants only cause brief upset — a raised bed or a fenced corner usually keeps curious pets clear.
Colorado Cooperative Extension
For Colorado-specific cultivar recommendations, planting calendars, and pest pressure for siver ironbark, the canonical source is Colorado State University Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Siver Ironbark native to Colorado?
No — the USDA PLANTS Database lists Siver Ironbark 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 Siver Ironbark in Colorado
When can I plant Siver Ironbark 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). No sourced winter-hardiness floor is on record for Siver Ironbark, so this page does not state whether it overwinters in Colorado and publishes no fall establishment date for it — check its hardiness range against your own zone before treating it as a permanent planting.
What hardiness zone is Siver Ironbark grown in across Colorado?
Colorado spans USDA hardiness zones 3b-7b (USDA ARS PHZM 2023). Siver Ironbark carries a range of zones 9-11, 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). Siver Ironbark needs 150+ 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 Siver Ironbark native to Colorado?
No — the USDA PLANTS Database lists Siver Ironbark 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 Siver Ironbark in Colorado?
Siver Ironbark prefers pH 5.5-7.5 and well (dry spells) drainage (FAO ECOCROP). 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 Siver Ironbark actually grow on my specific land in Colorado?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores siver ironbark 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 siver ironbark against your parcel's exact soil, sun, drainage, and frost data — not zone averages.
Three things about your lot that Colorado siver ironbark 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 siver ironbark average
Explore More
Growing guides for siver ironbark
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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