Conditional — Some Areas
cast iron plant (zones 7-11) reaches only part of Illinois (5a-7b): zone 7 are in range, so where you are in the state decides it.
Your yard isn't the whole zone.
Illinois spans zones 5a-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 cast iron plant against your parcel's actual hardiness, soil, and sun.
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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
Illinois Has
- USDA Zones: 5a-7b
- Last Hard Freeze (28°F): Mar 29 - Apr 8
- First Hard Freeze (28°F): Oct 28 - Nov 5
- Annual Rainfall: 34-48 inches
- Common Soils: Prairie loam, Silt loam, Clay loam
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 Illinois
The frost window
Across Illinois, the last spring hard freeze (28°F) clears between Mar 29 and Apr 8, and the first fall hard freeze lands between Oct 28 and Nov 5 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 203-day window you can count on — up to 221 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
Illinois runs as cold as USDA zone 5, 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 Illinois; check its hardiness range against that before treating it as long-lived ground.
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 survives winter in part of Illinois, not across all of it. Whether the season is long enough — and warm enough — is a different question.
Frost-free days
Cast Iron Plant wants 120+ frost-free days; a typical Illinois site sees ~212 (NOAA Regional Climate Centers ACIS, 1991-2020). That leaves comfortable headroom for succession planting.
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 Illinois's prairie loam and silt loam — 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 Illinois soil test result is a problem or just a number. Drainage matters: this plant wants well (dry spells). If your Illinois site is heavier clay or sits in a low spot, raised beds or amendment with compost solve it.
Plant pH and drainage requirements from USDA PLANTS Database. Illinois soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
Cast Iron Plant in Illinois — Quick Answer
- Verdict: Conditional — Some Areas
- Plant Zones: 7-11 (USDA PLANTS Database)
- State Zones: 5a-7b (USDA ARS PHZM 2023)
- Growing Season: 28°F hard freezes about Mar 29 - Apr 8 to Oct 28 - Nov 5 (NOAA Regional Climate Centers ACIS, 1991-2020)
What Else to Consider for Cast Iron Plant in Illinois
Zone compatibility tells you about winter cold survival — but Illinois growers also need to think about:
Heavy clay soils in northern IL drain poorly
Extreme temperature swings between summer and winter
Japanese beetles are a major garden pest
What to actually do about these — and the constraints that apply to every Illinois grower, not just cast iron plant — is covered on the Illinois growing guide.
Timing shifts within Illinois
Cast Iron Plant needs about 120 days of frost-free season to complete its cycle. Even Jo Daviess County — Illinois’s shortest season at 177 frost-free days — clears that by 57 days, so season length isn’t the constraint anywhere in Illinois. What varies is your own parcel.
Across Illinois’s 102 counties the frost-free season runs from 177 days in Jo Daviess County to 244 days in Alexander County — a spread of 67 days, which is why a single statewide planting date misleads. The median Illinois county sees 212 frost-free days, leaving cast iron plant about 92 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.
Where in Illinois Fits Best
Even within Illinois's zones 5a-7b, county microclimates differ enough to change what thrives. These counties carry the closest zone match for cast iron plant (USDA PHZM 2023):
Illinois Cooperative Extension
For Illinois-specific cultivar recommendations, planting calendars, and pest pressure for cast iron plant, the canonical source is University of Illinois Extension. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Cast Iron Plant native to Illinois?
No — the USDA PLANTS Database lists Cast Iron Plant as introduced rather than native in the Lower 48, so it is not part of Illinois's native flora. It grows here as a garden plant; pairing it with a few Illinois natives keeps local pollinators fed too.
Looking for plants that belong here? The Illinois 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 Illinois
When can I plant Cast Iron Plant in Illinois?
Illinois's last spring hard freeze (28°F) clears between Mar 29 and Apr 8, and the first fall hard freeze lands between Oct 28 and Nov 5 (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 Illinois 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 Illinois?
Illinois spans USDA hardiness zones 5a-7b (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 Illinois site have?
A typical Illinois site sees ~212 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.
Is Cast Iron Plant native to Illinois?
No — the USDA PLANTS Database lists Cast Iron Plant as introduced rather than native in the Lower 48, so it is not part of Illinois's native flora. It grows here as a garden plant; pairing it with a few Illinois natives keeps local pollinators fed too.
How should I amend the soil for Cast Iron Plant in Illinois?
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 Illinois 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 Illinois?
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 Illinois
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 Illinois cast iron plant averages can't show:
We read public map data around Illinois — soil, climate, flood, and parcel records. How we handle your address.
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