Conditional — Some Areas
ironwood (zones 10-13) reaches only part of California (4b-11a): zones 10-11 are in range, so where you are in the state decides it.
Ironwood in California — the short answer
Ironwood in California: hardy in zones 10–13, full sun, against a state median of ~317 frost-free days.
- Plant zones
- 10-13
- Manual horticultural correction (source not retained); NC State Extension and Missouri Botanical Garden, with the remaining rows from other published sources or Growable Ground compilation
- California zones
- 4b-11a
- USDA PHZM 2023
- Frost window
- Jan 19 - Mar 19 – Nov 29 - Dec 12
- NOAA RCC ACIS 1991-2020, 28°F
- Shortest county season
- 108 days
- NOAA Regional Climate Centers ACIS, 1991-2020, county 28°F killing freeze
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS and university Extension sources — per-field provenance varies and is published with the dataset.
Your yard isn't the whole zone.
California spans zones 4b-11a, 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 ironwood against your parcel's actual hardiness, soil, and sun.
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California is not one climate
California’s winter-low ladder runs across 58 counties, from zone 6 in Mono County to zone 10.5 in San Francisco County. That is 45°F between the two counties’ average coldest night — the difference between ironwood overwintering in the ground and coming under cover.
The middle half of 58 counties runs 9–9.5 zones, with a median of 9.5 zones — a 4.5-zone span end to end.
Measured across 58 California counties. USDA PHZM 2023, per county. Quartiles are order statistics — every figure shown is a value a real county recorded, not an interpolation. Your own site sits somewhere inside this range; a parcel report resolves where.
Where in California Fits Best
Ironwood is rated for zones 10–13 — a cold floor USDA puts at 30 to 40°F. Against California’s 58 counties that band holds in 7 of them, and the line runs at Los Angeles County (zone 10a), the coldest county still inside it.
Inside zones 10–13
7 counties
- San Mateo County · zone 10a
- Santa Barbara County · zone 10a
- Ventura County · zone 10a
3 of 7 shown
Below the zone 10 floor
51 counties
- Mono County · zone 6a
- Alpine County · zone 6b
- Lassen County · zone 7a
3 of 51 shown
Winter cold is the limit here — the same plant works in a container that comes under cover.
County hardiness zones: USDA Plant Hardiness Zone Map (PHZM) 2023, per county. A zone is a 10°F band of average annual extreme minimum temperature, so the floor above is the band the plant's rating denotes, not a single measured kill point. Slope, shelter and cold-air drainage routinely move a site half a zone either way from its county figure — which is the part only your address resolves.
Even within California's zones 4b-11a, county microclimates differ enough to change what thrives. These counties carry the closest zone match for ironwood (USDA PHZM 2023):
Zone Comparison
Ironwood Needs
- USDA Zones: 10-13
- Frost-Free Days: 365+
- Sun: Full Sun
- Drought tolerance: High
California Has
- USDA Zones: 4b-11a
- Last Hard Freeze (28°F): Jan 19 - Mar 19
- First Hard Freeze (28°F): Nov 29 - Dec 12
- Annual Rainfall: 5-80 inches
- Common Soils: Alluvial clay, Sandy loam, Adobe clay
Plant Zone Range (zones 10-13)
Preferred Soil pH
Plant data: Growable Ground Plant Database, compiled from FAO ECOCROP, USDA PLANTS, and university Extension sources — per-field provenance varies and is published with the dataset. State data: USDA ARS PHZM 2023, NOAA Climate Normals, NRCS SSURGO.
When to Plant Ironwood in California
The frost window
Across California, the last spring hard freeze (28°F) clears between Jan 19 and Mar 19, and the first fall hard freeze lands between Nov 29 and Dec 12 (NOAA Regional Climate Centers ACIS, 1991-2020). Counting from the latest last freeze to the earliest first freeze, that's a 255-day window you can count on — up to 327 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
California runs as cold as USDA zone 4, and no sourced winter-hardiness floor is on record for ironwood, so this page does not state whether it overwinters here — and gives no fall establishment date on the strength of a guess. Whether ironwood is a permanent planting or a replant-every-spring crop for you turns on your own zone within California; check its hardiness range against that before treating it as long-lived ground.
Frost window: NOAA Climate Normals 1991-2020. Plant timing fields: Growable Ground Plant Database — frost-free days from FAO ECOCROP, days to harvest from university Extension production guides, and growing-degree-day and chill-hour minimums that are compilation estimates rather than published figures. 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 ironwood survives winter in part of California, not across all of it. Whether the season is long enough — and warm enough — is a different question.
Frost-free days
Ironwood wants 365+ frost-free days; a typical California site sees ~317 (NOAA Regional Climate Centers ACIS, 1991-2020) — 48 days short of what it needs. A typical California season does not carry this crop to harvest outdoors, and a gap that size is not one an earlier sowing date or a faster cultivar closes; growing it here means growing it under cover.
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
Ironwood likes near-neutral soil (pH 6.8-8.6). That's the common-ground band across California's alluvial clay and sandy loam — a soil test confirms it for your site.
Plant pH requirements: FAO ECOCROP and USDA PLANTS, with the remaining rows from other published sources or Growable Ground compilation. Drainage: FAO ECOCROP, with the remaining rows from other published sources or Growable Ground compilation. California soil profile from USDA NRCS SSURGO. Site-specific verification: a 30-minute soil test from your local Extension lab.
What Else to Consider for Ironwood in California
Zone compatibility tells you about winter cold survival — but California growers also need to think about:
Drought is a persistent challenge — irrigation is essential in most regions
Wildfire risk affects rural and foothill properties
Adobe clay soils in valleys drain poorly without amendment
Wide climate variation means plant selection is highly location-specific
What to actually do about these — and the constraints that apply to every California grower, not just ironwood — is covered on the California growing guide.
Timing shifts within California
Ironwood needs about 365 days of frost-free season to complete its cycle. 5 of California’s 57 counties give it that outright. The tightest that does is Los Angeles County at 365 frost-free days — 0 to spare. Below that line you are transplanting or extending the season, not direct-sowing.
Across California’s 57 counties the frost-free season runs from 108 days in Mono County to 365 days in Ventura County — a spread of 257 days, which is why a single statewide planting date misleads. The median California county sees 317 frost-free days, which is 48 days short of what ironwood needs — so on a typical California parcel this is a transplant-or-protect crop, not a direct-sow one.
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"). Ironwood's 365-day requirement: USDA PLANTS / Growable Ground plant database (minimum frost-free days). A parcel report resolves your address's own frost dates.
California Cooperative Extension
For California-specific cultivar recommendations, planting calendars, and pest pressure for ironwood, the canonical source is UC Agriculture and Natural Resources. Their fact sheets carry the local trial data we can't generalize across 50 states.
Is Ironwood native to California?
Yes — the USDA PLANTS Database (accessed 2026-07-01) documents Ironwood as native to California. Planting it supports the pollinators and wildlife that evolved alongside it.
Native-range data: USDA PLANTS Database state-distribution records, accessed 2026-07-01.
Common Questions About Growing Ironwood in California
When can I plant Ironwood in California?
California's last spring hard freeze (28°F) clears between Jan 19 and Mar 19, and the first fall hard freeze lands between Nov 29 and Dec 12 (NOAA Regional Climate Centers ACIS, 1991-2020). No sourced winter-hardiness floor is on record for Ironwood, so this page does not state whether it overwinters in California 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 Ironwood grown in across California?
California spans USDA hardiness zones 4b-11a (USDA ARS PHZM 2023). Ironwood carries a range of zones 10-13, so the overlap zones are where outdoor growing is most reliable.
How many frost-free days does a typical California site have?
A typical California site sees ~317 frost-free days per year (derived from NOAA Regional Climate Centers ACIS, 1991-2020). Ironwood needs 365+ frost-free days, so check whether your local microclimate runs above or below the state average before settling on a planting date.
Is Ironwood native to California?
Yes — the USDA PLANTS Database (accessed 2026-07-01) documents Ironwood as native to California. Planting it supports the pollinators and wildlife that evolved alongside it.
How should I amend the soil for Ironwood in California?
Ironwood prefers pH 6.8-8.6 (USDA PLANTS). That sits in the common-ground band across California soils — a 30-minute soil test from a local Extension lab confirms it for your specific site.
Will Ironwood actually grow on my specific land in California?
State-level zone + climate data is a sketch. A Growable Ground parcel report scores ironwood 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 California
State-level data is a sketch. Your Growable Ground report scores ironwood against your parcel's exact soil, sun, drainage, and frost data — not zone averages.
Three things about your lot that California ironwood averages can't show:
We read public map data around California — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the California ironwood average
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