Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated

Planting zone for 94528 — Diablo, CA

ZIP code 94528 (Diablo, Contra Costa County) is in USDA zone 9b: the coldest winter night averages 25 to 30 °F. The median last spring frost is March 8 and the first fall frost December 22.

Where ZIP 94528 is

Outlined: the ZIP code area · tap the map for the zone at any spot · scroll or pinch to zoom

Not the official USDA Plant Hardiness Zone Map. Zones come from the 2023 USDA hardiness-zone data by the PRISM Group, Oregon State University; we re-mapped and recolored them. The official map is at planthardiness.ars.usda.gov.

Zone9b25 to 30 °F
Coldest night−3.9 to −1.1 °C
Last frostMar 8
First frostDec 22
2012 zone9b

Same zone as on the 2012 map. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 9b means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Diablo, CA…

Forecast data: National Weather Service (api.weather.gov), public domain. This is not a warning service — for official frost and freeze warnings see weather.gov.

Frost dates in Diablo, CA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMarch 8median — 50% chance of a later frost
Safe for tender plantsApril 15only 10% chance of frost after
First fall frostDecember 22median — 10% chance by Nov 23
Growing season282 daysfrost-free, median

A hard freeze (28 °F) usually ends around February 8 and returns around January 16.

From the NOAA station Mt Diablo Junction, CA — 3 miles away, elevation 2,169 ft. NOAA NCEI U.S. Climate Normals 1991–2020, 32 °F threshold. Low spots and valleys can freeze later in spring than the station.

When to plant vegetables in 94528

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Jan 25 – Feb 8
SpinachSow outdoors · Jan 25 – Feb 15
Broccoli, cabbageStart indoors · Dec 28 – Jan 11
Broccoli, cabbageTransplant · Feb 15 – Mar 1
Onion setsPlant outdoors · Feb 8 – Feb 22
PotatoesPlant outdoors · Feb 15 – Mar 1
LettuceSow outdoors · Feb 8 – Feb 22
Carrots, beetsSow outdoors · Feb 15 – Mar 1
TomatoesStart indoors · Jan 11 – Jan 25
TomatoesTransplant · Mar 15 – Mar 29
Peppers, eggplantStart indoors · Dec 28 – Jan 11
Peppers, eggplantTransplant · Mar 22 – Apr 5
BeansSow outdoors · Mar 15 – Mar 29
Sweet cornSow outdoors · Mar 15 – Mar 29
Cucumbers, squashSow outdoors · Mar 15 – Mar 29
Fall lettuce, spinachSow outdoors · Oct 27 – Nov 10
GarlicPlant cloves · Dec 8 – Jan 12
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Mar 8) and first fall frost (Dec 22) with the rules of thumb U.S. extension services publish. Cool-season crops take a light frost; warm-season crops do best once the soil is above about 60 °F.

When spring arrived in 2026

In 2026, spring leaf-out near 94528 came on Jan 30 — about the same time as the 1991–2020 average (Jan 31).

JanFebMarAprMayJun

The Spring Index marks the day when accumulated warmth is enough for lilac and honeysuckle to leaf out — a widely used signal that the growing season has started. Source: USA National Phenology Network, www.usanpn.org — Spring Indices, first leaf, 2026 (data as of 2026-09-08) and 30-year average 1991–2020 (CC BY 4.0).

Soil in Diablo, CA

The soil mapped at the center of ZIP 94528 is Los Osos — clay loam, well drained.

TextureClay loamholds water and nutrients; slow to drain and warm up
pH6.4slightly acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter3%moderate
Hydrologic groupDwater soaks in very slowly
Farmland classNot prime farmlandUSDA classification

Map unit: Los Osos clay loam, 30 to 50 percent slopes.

Soil Survey Staff, Natural Resources Conservation Service, USDA. Soil Survey Geographic (SSURGO) Database, via Soil Data Access, accessed 2026-10-29. The soil at the center point of ZIP 94528; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Diablo, CA

Summer high83 °Faverage high, July
Winter low41 °Faverage low, December
Hot days27days a year at 90 °F or more
Freezing nights19nights a year at 32 °F or less
Growing degree days3,645base 50 °F — the season’s usable heat
Longest day14 h 47 mindaylight on June 21
Shortest day9 h 32 mindaylight on December 21
Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully
Precipitation26.1 inper year, rain and melted snow
Snowfall1 inper year

Average high and low, by month

30°40°50°60°70°80°90°32 °FJanuary: average high 54 °F, low 42 °F 54°42° JanFebruary: average high 55 °F, low 42 °F 55°42° FebMarch: average high 58 °F, low 43 °F 58°43° MarApril: average high 62 °F, low 44 °F 62°44° AprMay: average high 68 °F, low 49 °F 68°49° MayJune: average high 76 °F, low 55 °F 76°55° JunJuly: average high 83 °F, low 62 °F 83°62° JulAugust: average high 83 °F, low 61 °F 83°61° AugSeptember: average high 80 °F, low 60 °F 80°60° SepOctober: average high 72 °F, low 54 °F 72°54° OctNovember: average high 60 °F, low 46 °F 60°46° NovDecember: average high 53 °F, low 41 °F 53°41° Dec

Precipitation by month inches · ❄ = snowfall

0″1″2″3″4″5″6″January: 5.06 in precipitation, 0.1 in snowfall 5.1 JanFebruary: 4.90 in precipitation, 0.5 in snowfall 4.9❄1 FebMarch: 3.69 in precipitation, 0.1 in snowfall 3.7 MarApril: 1.92 in precipitation 1.9 AprMay: 1.13 in precipitation 1.1 MayJune: 0.27 in precipitation 0.3 JunJuly: 0.01 in precipitation 0.0 JulAugust: 0.03 in precipitation 0.0 AugSeptember: 0.12 in precipitation 0.1 SepOctober: 1.19 in precipitation 1.2 OctNovember: 2.65 in precipitation 2.6 NovDecember: 5.08 in precipitation 5.1 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Mt Diablo Junction, CA (3 mi away); precipitation: Mt Diablo Junction, CA (3 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 9b.

945269b
Danville, CA · 2 mi
945079b
Alamo, CA · 3 mi
945069b
Danville, CA · 4 mi
945709b
Moraga, CA · 4 mi
945179b
Clayton, CA · 5 mi
945839b
San Ramon, CA · 5 mi
945969b
Walnut Creek, CA · 5 mi
945829b
San Ramon, CA · 6 mi

California planting zones →

Questions about 94528

What planting zone is ZIP code 94528?

ZIP code 94528 (Diablo, CA) is in USDA hardiness zone 9b on the 2023 map: the coldest night of an average winter reaches 25 to 30 °F (−3.9 to −1.1 °C).

When is the last frost in Diablo, CA?

The median last spring frost (32 °F) near 94528 is March 8; after April 15 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Mt Diablo Junction station.

When is the first frost in Diablo, CA?

The median first fall frost near 94528 is December 22, giving a median frost-free growing season of 282 days.

When should I plant tomatoes in 94528?

Start tomato seeds indoors around Jan 11 – Jan 25 and set transplants outside around Mar 15 – Mar 29, one to three weeks after the median last frost.

How much rain and snow does 94528 get?

About 26.1 inches of precipitation a year and 1 inches of snowfall (NOAA 1991–2020 normals, Mt Diablo Junction station).

How hot do summers get in Diablo, CA?

The average high peaks at 83 °F in July, with about 27 days a year at 90 °F or more.

What kind of soil does 94528 have?

The USDA soil survey maps Los Osos at the center of ZIP 94528: clay loam, well drained, pH about 6.4 (slightly acidic).

Did spring come early in Diablo, CA in 2026?

Spring leaf-out near 94528 came on Jan 30 in 2026, about the same as the 1991–2020 average (Jan 31), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Diablo, CA?

Intermediate onions: the longest day near 94528 is 14 h 47 min, so day-neutral types bulb best; long-day varieties may not bulb fully.