Diablo, California, sits in USDA hardiness zone 9b — room for a real mix of vegetables, fruit, and perennials matched to the local frost calendar.
On paper, okra, meyer lemon, california poppy, and artichoke all suit these conditions — on the ground, soil, sun, and drainage make the final call.
Even in Diablo, no two yards are alike.
A low spot, a south-facing slope, or a stand of trees moves the frost date and sun across a single Diablo lot. Enter your address and we'll score 1,086 plants against your land's actual soil, sun, and frost.
We read public map data around Diablo — soil, climate, flood, and parcel records. How we handle your address.
No card required · your full report in seconds
Diablo — the short answer
Diablo, California: USDA zone 9b, about 332 days between hard freezes, across 873 acres of town.
- USDA zone
- 9b
- USDA PHZM 2023
- Last hard freeze (28°F)
- Jan 16
- Contra Costa County normal — NOAA 1991–2020, nearest to this town
- First hard freeze (28°F)
- Dec 14
- Contra Costa County normal — NOAA 1991–2020, nearest to this town
- Days between freezes
- ~332
- Contra Costa County normal — NOAA 1991–2020, nearest to this town
- Town area
- 873 acres
- US Census TIGER/Line 2025
Every figure names the dataset it came from. No 30-year normal resolves at this town’s own centroid, so the hard-freeze dates are Contra Costa County’s — the closest measured record, labelled as such rather than passed off as local.
Hardiness Zone Range
Zone 9b is Diablo's average, not your lot's — a low corner or a tree line moves it. Against Diablo's 332-day season, see what your own ground reads.
Soil varies lot by lot — soil types explained.
What this means for growing in Diablo
Each reading below is measured for Diablo itself — here is what it means for a garden here, and the move an experienced grower makes about it.
Season length
From county recordsDiablo runs about 332 days between hard freezes (read from Contra Costa County, the closest normal to Diablo) — close to year-round growing.
Frost is rarely the limit here; winter plantings are as real as summer ones, and the harder question is which crops shrug off the summer heat.
Grow cool-season crops through the mild winter, and choose heat-tolerant varieties for the summer beds.
NOAA 1991–2020 Climate Normals
Winter hardiness
MeasuredDiablo sits in USDA hardiness zone 9b.
The zone sets which perennials, shrubs, and fruit trees live through an average winter here — a plant rated for a colder zone is a safe bet, one rated warmer needs winter protection or gets treated as an annual.
Match perennials and fruit to the zone before you buy, so nothing you plant is a gamble against the cold.
USDA PHZM 2023
The kind of country Diablo is in
Diablo sits in the Central California Foothills and Coastal Mountains — EPA Level III, inside the wider Mediterranean California. That is read at Diablo's own centroid rather than borrowed from Contra Costa County.
What Grows in Diablo
Every plant below cleared Diablo's own zone 9b and its 332-day season. Each card opens that plant's California profile. How we score.
Is it too late to plant in Diablo?
For most of the year, no — what changes is which crops still fit the days remaining. Cool-season crops can go in from around Jan 1; tender transplants wait until two to three weeks after the last 28°F hard freeze, which lands near Jan 16 (NOAA 1991–2020 climate normals); and from midsummer, planting counts back from the first fall freeze around Dec 14 — long-season crops need about 90 days of runway, quick greens only 30. And with a calendar this mild, the honest answer is that planting barely stops — winter opens seasons colder regions never see.
Documented around Diablo
Spotted lanternfly documented 17 miles from Diablo, first recorded 2023. What to watch for.
What's on the federal record near Diablo
Closest first, within ~10 miles of Diablo. Listing isn't leaking, and distance isn't exposure.
- BILL LETTISBrownfields · 200 ft
- DIABLO COUNTRY CLUBBrownfields · 1700 ft
- TIM COTTONBrownfields · 2000 ft
- JENNA BARROSBrownfields · 2300 ft
- MATT HALLBrownfields · 0.5 mi
- 111 EL CENTROBrownfields · 0.6 mi
- THE ATHENIAN SCHOOLBrownfields · 0.6 mi
- Mount ZionMining · 0.9 mi
- PAUL KROUTBrownfields · 1.0 mi
- TERI KOSCINSKIBrownfields · 1.0 mi
- PAT MCMAHONBrownfields · 1.1 mi
- AT&T MOBILITYBrownfields · 1.2 mi
6 more sites are on the record inside the same radius. See them on the map
Measured from Diablo's center, not your beds. Run your own address for what's upwind of you specifically.
Environmental Intelligence
The counts behind that headline — every source type on the record near Diablo.
Total Sites
2,313
within ~10 miles of Diablo
Risk Level
Elevated
Highest-severity
4 Superfund sites (none NPL-listed)
Sources Checked
within ~10 miles of Diablo
Severity Distribution
within ~10 miles of Diablo
Highest-Severity Sites
A note from Gnorman
What an experienced grower watches for around here
In and around Diablo, Brownfields runs higher than the national average — 2,067 sites within ~10 miles of Diablo. Knowing it is half the work — and it's nothing a thoughtful grower can't plan for.
Brownfields: Brownfield sites are former commercial or industrial properties where legacy soil contamination (heavy metals, PAHs, petroleum compounds) may persist.
Check EPA brownfield remediation status — many sites have completed cleanup with institutional controls.
Check your specific parcel in Diablo
Get exact proximity distances to contamination sources for your specific parcel — plus soil, sun, drainage, and 1,086 plant recommendations.
25+ data sources read at your address, not the Diablo average
Your land is one point inside Diablo
Everything above resolves to Diablo as a whole — one PHZM band, read across 873 acres, with hard-freeze dates borrowed from Contra Costa County’s record because no 30-year normal resolves at this town’s own centroid. Your parcel is one point inside that footprint. Slope, tree cover, a low spot that pools cold air, and what is in the soil are all resolved at the address, and none of them are visible at town scale. That is the honest boundary of this page, and the reason the next step is a reading of your own ground rather than a better local average.
Your Lot vs the Diablo Average
Diablo Average
- ●USDA Zones 9b, averaged across Diablo
- ●One soil character for all of Diablo
- ●Frost dates read at Diablo's scale: Jan 16 to Dec 14
YOUR Parcel
- ✓Your exact zone, not Diablo's 9b
- ✓Your SSURGO soil type & pH
- ✓Your sun exposure, cast in 3D
See MY Growing Report
Read your specific parcel in Diablo
Pull a site-specific report for your exact address in Diablo, California — soil, sun, drainage, frost risk, and scored plant recommendations.
Three things about your lot that Diablo averages can't show:
We read public map data around Diablo — soil, climate, flood, and parcel records. How we handle your address.
25+ data sources read at your address, not the Diablo average
How we know this about Diablo
The datasets: hardiness from the USDA ARS Plant Hardiness Zone Map 2023, read at this town’s own centroid; hard-freeze dates from NOAA 1991–2020 28°F normals at Contra Costa County’s record, because none resolves here; boundaries and acreage from US Census TIGER/Line 2025; the contamination records from the federal sources named in their beats above. Soil is not read at town grain anywhere on this page — there is no town-resolution survey in our data, which is why you will find no soil grid here rather than a statewide one wearing a local label. The resolution ledger below names every figure this page inherited.
Common questions about Diablo
When does frost risk typically end in Diablo?
The last hard freeze (28°F) in Diablo typically lands around Jan 16, per NOAA 1991–2020 climate normals — earlier than the light-frost dates most charts quote. That marks the hard freeze, not the last light frost — lighter frosts keep biting past it, which is why tender transplants here wait until about Feb 6.
When is the first frost in Diablo?
The first hard freeze (28°F) in Diablo typically arrives around Dec 14, per NOAA 1991–2020 climate normals — the point tender summer crops finish. Lighter frosts run ahead of it, near Nov 30, so the watching starts about Nov 14 — about 332 days after the spring freeze cleared.
How long is the growing season in Diablo?
Measured between 28°F hard freezes, Diablo sees about 332 frost-free days — roughly Jan 16 through Dec 14, per NOAA 1991–2020 climate normals. Tender crops work a shorter one: nearer Feb 6 to Nov 30, about 297 days.
Is the soil safe to grow vegetables in Diablo?
The federal record around Diablo is a meaningful one — 2,313 documented sites, mostly Brownfields entries (2,067), the nearest Brownfields record under a tenth of a mile out — so a soil test before new food beds is a sensible precaution here, not a reason to hold back from growing. Remember that proximity to a documented site is information, not a diagnosis of any one yard; the contamination map shows exactly what sits where.
How do I protect my plants from frost in Diablo?
From about Nov 14 the job here turns from planting to protecting — the last start even a 30-day crop can finish. As the season closes around the first 28°F hard freeze near Dec 14 (NOAA 1991–2020 climate normals), three moves buy time: row cover or an old sheet over tender plants on still, clear nights; water the soil the afternoon before a freeze, so it holds warmth overnight; and harvest tomatoes, peppers and basil before Dec 14. Hardy greens and roots shrug off light frost and sweeten after it — leave them in.
Everything above is measured at Diablo's scale — its 9b band and its 332-day season. Your yard writes its own version of Diablo's numbers: we read soil, sun, drainage and frost at your exact address. Try it for 14 days — no card required.
What this page reads at Diablo’s own scale
6 of the readings below were taken at Diablo's own scale — the 873 acres it covers and 2,313 documented sites around it; 5 are borrowed from a wider read, named and linked below.
Measured here
- Diablo's own USDA hardiness band, zones 9bUSDA PHZM 2023
- 2,313 documented contamination sites on the federal record near Diablo, the closest Brownfields record under a tenth of a mile out from the town centreFederal environmental registers
- Diablo's boundary and the 873 acres inside itUS Census TIGER/Line 2025
- Which of 1,086 plants clear zones 9b and a ~332-day seasonGrowable Ground crop-matching engine
- The ecoregion at Diablo's own centroid, the Central California Foothills and Coastal MountainsUS EPA Level III Ecoregions
- A documented spotted-lanternfly record within reach of Diablolydemapr (De Bona & Helmus), 10 km survey lattice
Inherited from a wider read
- The 28°F hard-freeze dates — Diablo has no normal of its own, so its county's is the closest measured readfrom Contra Costa County
- Soil character — no town-scale survey exists, so Diablo's 873 acres read through its county's SSURGO summaryfrom Contra Costa County
- Growing-degree days and winter chill — measured county-wide only, so the county page carries themfrom Contra Costa County
- The growing challenges every California grower plans around — written at state scale, so they stay on the state guidefrom California
- Butterfly and beetle records, and the farmland pesticide estimate — all published county-widefrom Contra Costa County
What this page can’t tell you
- The soil pH, drainage, sun hours, and frost pockets on any one lot among Diablo's 873 acreswhy one lot differs from the next →
- Which side of Diablo you're on — a low spot frosts first and a south wall warms early, and one normal is being asked to speak for all 873 acresreading your own microclimate →
- How close that record sits to YOUR lot — the closest Brownfields record under a tenth of a mile out, measured from the town centrehow we read the record →
- Where cold air settles, which slopes face the sun, and which way the wind runs on your own ground — measured at your address, and averaged away by any Diablo-wide figureread your own land →