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

Planting zone for 94503 — American Canyon, CA

ZIP code 94503 (American Canyon, Napa County) is in USDA zone 9b: the coldest winter night averages 25 to 30 °F. The median last spring frost is March 5 and the first fall frost November 21.

Where ZIP 94503 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 5
First frostNov 21
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 American Canyon, 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 American Canyon, CA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostMarch 5median — 50% chance of a later frost
Safe for tender plantsApril 16only 10% chance of frost after
First fall frostNovember 21median — 10% chance by Nov 3
Growing season256 daysfrost-free, median

A hard freeze (28 °F) usually ends around January 23 and returns around December 5.

From the NOAA station Napa Co. Airport, CA — 4 miles away, elevation 13 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 94503

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Jan 22 – Feb 5
SpinachSow outdoors · Jan 22 – Feb 12
Broccoli, cabbageStart indoors · Dec 25 – Jan 8
Broccoli, cabbageTransplant · Feb 12 – Feb 26
Onion setsPlant outdoors · Feb 5 – Feb 19
PotatoesPlant outdoors · Feb 12 – Feb 26
LettuceSow outdoors · Feb 5 – Feb 19
Carrots, beetsSow outdoors · Feb 12 – Feb 26
TomatoesStart indoors · Jan 8 – Jan 22
TomatoesTransplant · Mar 12 – Mar 26
Peppers, eggplantStart indoors · Dec 25 – Jan 8
Peppers, eggplantTransplant · Mar 19 – Apr 2
BeansSow outdoors · Mar 12 – Mar 26
Sweet cornSow outdoors · Mar 12 – Mar 26
Cucumbers, squashSow outdoors · Mar 12 – Mar 26
Fall lettuce, spinachSow outdoors · Sep 26 – Oct 10
GarlicPlant cloves · Nov 7 – Dec 12
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Mar 5) and first fall frost (Nov 21) 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 94503 came on Jan 29 — about the same time as the 1991–2020 average (Jan 30).

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 American Canyon, CA

The soil mapped at the center of ZIP 94503 is Fagan — clay loam, well drained.

TextureClay loamholds water and nutrients; slow to drain and warm up
pH6.1slightly acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter2%moderate
Hydrologic groupCwater soaks in slowly
Farmland classNot prime farmlandUSDA classification

Map unit: Fagan clay loam, 30 to 50 percent slopes, slipped.

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 94503; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in American Canyon, CA

Summer high80 °Faverage high, August
Winter low38 °Faverage low, January
Hot days16days a year at 90 °F or more
Freezing nights34nights a year at 32 °F or less
Growing degree days3,153base 50 °F — the season’s usable heat
Longest day14 h 49 mindaylight on June 21
Shortest day9 h 30 mindaylight on December 21
Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully
Precipitation20.2 inper year, rain and melted snow

Average high and low, by month

30°40°50°60°70°80°90°32 °FJanuary: average high 57 °F, low 38 °F 57°38° JanFebruary: average high 61 °F, low 40 °F 61°40° FebMarch: average high 64 °F, low 42 °F 64°42° MarApril: average high 68 °F, low 44 °F 68°44° AprMay: average high 73 °F, low 48 °F 73°48° MayJune: average high 78 °F, low 52 °F 78°52° JunJuly: average high 79 °F, low 55 °F 79°55° JulAugust: average high 80 °F, low 54 °F 80°54° AugSeptember: average high 80 °F, low 51 °F 80°51° SepOctober: average high 75 °F, low 46 °F 75°46° OctNovember: average high 65 °F, low 40 °F 65°40° NovDecember: average high 57 °F, low 38 °F 57°38° Dec

Precipitation by month inches

0″1″2″3″4″5″January: 4.17 in precipitation 4.2 JanFebruary: 3.95 in precipitation 4.0 FebMarch: 3.01 in precipitation 3.0 MarApril: 1.26 in precipitation 1.3 AprMay: 0.80 in precipitation 0.8 MayJune: 0.22 in precipitation 0.2 JunJuly: 0.01 in precipitation 0.0 JulAugust: 0.02 in precipitation 0.0 AugSeptember: 0.07 in precipitation 0.1 SepOctober: 0.93 in precipitation 0.9 OctNovember: 1.97 in precipitation 2.0 NovDecember: 3.82 in precipitation 3.8 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Napa Co. Airport, CA (4 mi away); precipitation: Napa Co. Airport, CA (4 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 9b.

945899b
Vallejo, CA · 3 mi
945919b
Vallejo, CA · 4 mi
945599b
Napa, CA · 6 mi
945909b
Vallejo, CA · 7 mi
945349b
Fairfield, CA · 7 mi
945929b
Vallejo, CA · 7 mi
945109b
Benicia, CA · 8 mi
945819b
Napa, CA · 8 mi

California planting zones →

Questions about 94503

What planting zone is ZIP code 94503?

ZIP code 94503 (American Canyon, 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 American Canyon, CA?

The median last spring frost (32 °F) near 94503 is March 5; after April 16 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Napa Co. Airport station.

When is the first frost in American Canyon, CA?

The median first fall frost near 94503 is November 21, giving a median frost-free growing season of 256 days.

When should I plant tomatoes in 94503?

Start tomato seeds indoors around Jan 8 – Jan 22 and set transplants outside around Mar 12 – Mar 26, one to three weeks after the median last frost.

How much rain and snow does 94503 get?

About 20.2 inches of precipitation a year (NOAA 1991–2020 normals, Napa Co. Airport station).

How hot do summers get in American Canyon, CA?

The average high peaks at 80 °F in August, with about 16 days a year at 90 °F or more.

What kind of soil does 94503 have?

The USDA soil survey maps Fagan at the center of ZIP 94503: clay loam, well drained, pH about 6.1 (slightly acidic).

Did spring come early in American Canyon, CA in 2026?

Spring leaf-out near 94503 came on Jan 29 in 2026, about the same as the 1991–2020 average (Jan 30), according to the USA National Phenology Network Spring Index.

What kind of onions grow in American Canyon, CA?

Intermediate onions: the longest day near 94503 is 14 h 49 min, so day-neutral types bulb best; long-day varieties may not bulb fully.