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

Planting zone for 91903 — Alpine, CA

ZIP code 91903 (Alpine, San Diego County) is in USDA zone 9b: the coldest winter night averages 25 to 30 °F. The median last spring frost is February 12 and the first fall frost December 24.

Where ZIP 91903 is

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 frostFeb 12
First frostDec 24
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 Alpine, 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 Alpine, CA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostFebruary 12median — 50% chance of a later frost
Safe for tender plantsMarch 20only 10% chance of frost after
First fall frostDecember 24median — 10% chance by Dec 2
Growing season317 daysfrost-free, median

A hard freeze (28 °F) usually ends around January 22 and returns around January 4.

From the NOAA station Alpine, CA — within 1 mile, elevation 1,696 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 91903

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Jan 1 – Jan 15
SpinachSow outdoors · Jan 1 – Jan 22
Broccoli, cabbageStart indoors · Dec 4 – Dec 18
Broccoli, cabbageTransplant · Jan 22 – Feb 5
Onion setsPlant outdoors · Jan 15 – Jan 29
PotatoesPlant outdoors · Jan 22 – Feb 5
LettuceSow outdoors · Jan 15 – Jan 29
Carrots, beetsSow outdoors · Jan 22 – Feb 5
TomatoesStart indoors · Dec 18 – Jan 1
TomatoesTransplant · Feb 19 – Mar 5
Peppers, eggplantStart indoors · Dec 4 – Dec 18
Peppers, eggplantTransplant · Feb 26 – Mar 12
BeansSow outdoors · Feb 19 – Mar 5
Sweet cornSow outdoors · Feb 19 – Mar 5
Cucumbers, squashSow outdoors · Feb 19 – Mar 5
Fall lettuce, spinachSow outdoors · Oct 29 – Nov 12
GarlicPlant cloves · Dec 10 – Jan 14
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Feb 12) and first fall frost (Dec 24) 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 91903 came on Jan 17 — 8 days earlier than the 1991–2020 average (Jan 25).

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 Alpine, CA

The soil mapped at the center of ZIP 91903 is Visalia — sandy loam, well drained.

TextureSandy loamdrains and warms fast; dries out and needs more feeding
pH6.3slightly acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter2.5%moderate
Hydrologic groupAwater soaks in fast
Farmland classPrime farmland if irrigatedUSDA classification

Map unit: Visalia sandy loam, 2 to 5 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 91903; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Alpine, CA

Summer high89 °Faverage high, August
Winter low42 °Faverage low, February
Hot days55days a year at 90 °F or more
Freezing nights3nights a year at 32 °F or less
Growing degree days4,757base 50 °F — the season’s usable heat
Longest day14 h 19 mindaylight on June 21
Shortest day9 h 59 mindaylight on December 21
Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully
Precipitation16.1 inper year, rain and melted snow

Average high and low, by month

30°40°50°60°70°80°90°100°32 °FJanuary: average high 64 °F, low 43 °F 64°43° JanFebruary: average high 64 °F, low 42 °F 64°42° FebMarch: average high 67 °F, low 44 °F 67°44° MarApril: average high 71 °F, low 47 °F 71°47° AprMay: average high 74 °F, low 51 °F 74°51° MayJune: average high 81 °F, low 54 °F 81°54° JunJuly: average high 87 °F, low 60 °F 87°60° JulAugust: average high 89 °F, low 62 °F 89°62° AugSeptember: average high 86 °F, low 60 °F 86°60° SepOctober: average high 78 °F, low 54 °F 78°54° OctNovember: average high 71 °F, low 48 °F 71°48° NovDecember: average high 63 °F, low 42 °F 63°42° Dec

Precipitation by month inches

0″1″2″3″4″January: 3.21 in precipitation 3.2 JanFebruary: 3.65 in precipitation 3.6 FebMarch: 2.73 in precipitation 2.7 MarApril: 1.18 in precipitation 1.2 AprMay: 0.57 in precipitation 0.6 MayJune: 0.13 in precipitation 0.1 JunJuly: 0.10 in precipitation 0.1 JulAugust: 0.12 in precipitation 0.1 AugSeptember: 0.25 in precipitation 0.3 SepOctober: 0.59 in precipitation 0.6 OctNovember: 1.48 in precipitation 1.5 NovDecember: 2.04 in precipitation 2.0 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Alpine, CA (within 1 mile); precipitation: Alpine, CA (within 1 mile). Daylight is calculated from latitude.

Nearby ZIP codes

6 of the 8 nearest ZIP codes are in a different zone — elevation and distance from water change winter lows quickly.

919019b
Alpine, CA · 3 mi
9202110a
El Cajon, CA · 6 mi
9201910b
El Cajon, CA · 7 mi
9204010a
Lakeside, CA · 9 mi
919169b
Descanso, CA · 9 mi
9193510a
Jamul, CA · 10 mi
919318b
Guatay, CA · 11 mi
9202210a
El Cajon, CA · 12 mi

All 2 ZIP codes in Alpine → · California planting zones →

Questions about 91903

What planting zone is ZIP code 91903?

ZIP code 91903 (Alpine, 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 Alpine, CA?

The median last spring frost (32 °F) near 91903 is February 12; after March 20 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Alpine station.

When is the first frost in Alpine, CA?

The median first fall frost near 91903 is December 24, giving a median frost-free growing season of 317 days.

When should I plant tomatoes in 91903?

Start tomato seeds indoors around Dec 18 – Jan 1 and set transplants outside around Feb 19 – Mar 5, one to three weeks after the median last frost.

How much rain and snow does 91903 get?

About 16.1 inches of precipitation a year (NOAA 1991–2020 normals, Alpine station).

How hot do summers get in Alpine, CA?

The average high peaks at 89 °F in August, with about 55 days a year at 90 °F or more.

What kind of soil does 91903 have?

The USDA soil survey maps Visalia at the center of ZIP 91903: sandy loam, well drained, pH about 6.3 (slightly acidic).

Did spring come early in Alpine, CA in 2026?

Spring leaf-out near 91903 came on Jan 17 in 2026, 8 days earlier than the 1991–2020 average (Jan 25), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Alpine, CA?

Intermediate onions: the longest day near 91903 is 14 h 19 min, so day-neutral types bulb best; long-day varieties may not bulb fully.