Unlikely — Zone Mismatch
cast iron plant (zones 7-11) does not overlap with Montana's zone range (3a-6b). Growing outdoors is unlikely without significant protection.
2 zones short — what would close it
Cast Iron Plant is hardy to zone 7, and the warmest ground in Montana is zone 5 — 2 zones short. Planted in open ground it will not survive a Montana winter. To grow it here, grow cast iron plant 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 7-11 (plant) against 3-5 (Montana). 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.
Montana spans zones 3a-6b, 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 cast iron plant against your parcel's actual hardiness, soil, and sun.
We read public map data for this spot — soil, climate, flood, and parcel records. How we handle your address.
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Zone Comparison
Cast Iron Plant Needs
- USDA Zones: 7-11
- Frost-Free Days: 120+
- Sun: Shade
- Mature size: 2 ft tall
- Drought tolerance: High
Montana Has
- USDA Zones: 3a-6b
- Last Hard Freeze (28°F): May 2 - May 13
- First Hard Freeze (28°F): Sep 28 - Oct 7
- Annual Rainfall: 10-20 inches
- Common Soils: Sandy loam, Clay, Glacial till
Plant Zone Range (zones 7-11)
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 Cast Iron Plant in Montana
The frost window
Across Montana, the last spring hard freeze (28°F) clears between May 2 and May 13, and the first fall hard freeze lands between Sep 28 and Oct 7 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 138-day window you can count on — up to 158 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
Montana runs as cold as USDA zone 3, and no sourced winter-hardiness floor is on record for cast iron plant, so this page does not state whether it overwinters here — and gives no fall establishment date on the strength of a guess. Whether cast iron plant is a permanent planting or a replant-every-spring crop for you turns on your own zone within Montana; check its hardiness range against that before treating it as long-lived ground.
Timing tuned to sub-state frost dates — Deer Lodge 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 cast iron plant does not survive a Montana 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 Montana under protection.
Frost-free days
Cast Iron Plant wants 120+ frost-free days; a typical Montana site sees ~151 (NOAA Regional Climate Centers ACIS, 1991-2020). That leaves a workable window — start indoors to bank time.
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
Cast Iron Plant likes near-neutral soil (pH 5.5-7). That's the common-ground band across Montana's sandy loam and clay — a soil test confirms it for your site. It will grow across that whole span, but it does best between pH 5.5 and 6.5 — worth knowing before you decide whether a Montana soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Montana site is heavier clay or sits in a low spot, raised beds or amendment with compost solve it.
Your land, not the state average
Montana's soils run mostly loam, but SSURGO maps the series, texture, and drainage under your exact parcel. What matters for cast iron plant is the horizon its roots actually work, not the surface. That map unit, not the state average, decides how cast iron plant performs.
Check your parcel → Source: USDA NRCS SSURGO.
Plant pH and drainage requirements from USDA PLANTS Database. Montana soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
Cast Iron Plant in Montana — Quick Answer
- Verdict: Unlikely — Zone Mismatch
- Plant Zones: 7-11 (USDA PLANTS Database)
- State Zones: 3a-6b (USDA ARS PHZM 2023)
- Growing Season: 28°F hard freezes about May 2 - May 13 to Sep 28 - Oct 7 (NOAA Regional Climate Centers ACIS, 1991-2020)
What Else to Consider for Cast Iron Plant in Montana
Zone compatibility tells you about winter cold survival — but Montana growers also need to think about:
Very short growing season (60-100 frost-free days)
Low rainfall requires irrigation in most areas
Extreme winter cold (-40F possible)
Growing cast iron plant here specifically
Montana's soils run mostly loam (Mollisols), and whether that suits cast iron plant'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 cast iron plant'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 Montana grower, not just cast iron plant — is covered on the Montana growing guide.
Timing shifts within Montana
Montana isn't one climate. In Deer Lodge County, the last hard freeze (28°F) holds until about Jun 21 — roughly 47 days later than the recorded state median — so plant cast iron plant to your county's window, not the statewide date.
Cast Iron Plant needs about 120 days of frost-free season to complete its cycle. 50 of Montana’s 56 counties give it that outright. The tightest that does is Jefferson County at 123 frost-free days — 3 to spare. Below that line you are transplanting or extending the season, not direct-sowing.
Across Montana’s 56 counties the frost-free season runs from 59 days in Deer Lodge County to 170 days in Petroleum County — a spread of 111 days, which is why a single statewide planting date misleads. The median Montana county sees 151 frost-free days, leaving cast iron plant about 31 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"). Cast Iron Plant's 120-day requirement: USDA PLANTS / Growable Ground plant database (minimum frost-free days). A parcel report resolves your address's own frost dates.
Montana Cooperative Extension
For Montana-specific cultivar recommendations, planting calendars, and pest pressure for cast iron plant, the canonical source is Montana State University Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Cast Iron Plant native to Montana?
No — the USDA PLANTS Database lists Cast Iron Plant as introduced rather than native in the Lower 48, so it is not part of Montana's native flora. It grows here as a garden plant; pairing it with a few Montana natives keeps local pollinators fed too.
Looking for plants that belong here? The Montana 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 Cast Iron Plant in Montana
When can I plant Cast Iron Plant in Montana?
Montana's last spring hard freeze (28°F) clears between May 2 and May 13, and the first fall hard freeze lands between Sep 28 and Oct 7 (NOAA Regional Climate Centers ACIS, 1991-2020). No sourced winter-hardiness floor is on record for Cast Iron Plant, so this page does not state whether it overwinters in Montana 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 Cast Iron Plant grown in across Montana?
Montana spans USDA hardiness zones 3a-6b (USDA ARS PHZM 2023). Cast Iron Plant carries a range of zones 7-11, so the overlap zones are where outdoor growing is most reliable.
How many frost-free days does a typical Montana site have?
A typical Montana site sees ~151 frost-free days per year (derived from NOAA Regional Climate Centers ACIS, 1991-2020). Cast Iron Plant needs 120+ 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 Deer Lodge, the freeze-free season runs shorter than the state average, so verify your own county's window.
Is Cast Iron Plant native to Montana?
No — the USDA PLANTS Database lists Cast Iron Plant as introduced rather than native in the Lower 48, so it is not part of Montana's native flora. It grows here as a garden plant; pairing it with a few Montana natives keeps local pollinators fed too.
How should I amend the soil for Cast Iron Plant in Montana?
Cast Iron Plant prefers pH 5.5-7 and well (dry spells) drainage (Growable Ground Plant Database). That sits in the common-ground band across Montana soils — a 30-minute soil test from a local Extension lab confirms it for your specific site.
Will Cast Iron Plant actually grow on my specific land in Montana?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores cast iron plant 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 Montana
State-level data is a sketch. Your Growable Ground report scores cast iron plant against your parcel's exact soil, sun, drainage, and frost data — not zone averages.
Three things about your lot that Montana cast iron plant averages can't show:
We read public map data around Montana — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Montana cast iron plant average
Explore More
Growing guides for cast iron plant
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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