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

Planting zone for 91023 — Mount Wilson, CA

ZIP code 91023 (Mount Wilson, Los Angeles County) is in USDA zone 9a: the coldest winter night averages 20 to 25 °F. The median last spring frost is April 28 and the first fall frost November 15.

Where ZIP 91023 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.

Zone9a20 to 25 °F
Coldest night−6.7 to −3.9 °C
Last frostApr 28
First frostNov 15
2012 zone8b

Was 8b on the 2012 map — half a zone warmer now. The zone is the one at the center of the ZIP code; hilly ZIPs can span two zones. What zone 9a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Mount Wilson, 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 Mount Wilson, CA

JanFebMarAprMayJunJulAugSepOctNovDec
Blue: frost possible · fading green: frost risk falling or rising (90% → 10%) · green: frost-free
Last spring frostApril 28median — 50% chance of a later frost
Safe for tender plantsMay 27only 10% chance of frost after
First fall frostNovember 15median — 10% chance by Oct 18
Growing season199 daysfrost-free, median

A hard freeze (28 °F) usually ends around April 8 and returns around December 4.

From the NOAA station Mt Wilson Cbs, CA — within 1 mile, elevation 5,709 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 91023

JanFebMarAprMayJunJulAugSepOctNovDec
PeasSow outdoors · Mar 17 – Mar 31
SpinachSow outdoors · Mar 17 – Apr 7
Broccoli, cabbageStart indoors · Feb 17 – Mar 3
Broccoli, cabbageTransplant · Apr 7 – Apr 21
Onion setsPlant outdoors · Mar 31 – Apr 14
PotatoesPlant outdoors · Apr 7 – Apr 21
LettuceSow outdoors · Mar 31 – Apr 14
Carrots, beetsSow outdoors · Apr 7 – Apr 21
TomatoesStart indoors · Mar 3 – Mar 17
TomatoesTransplant · May 5 – May 19
Peppers, eggplantStart indoors · Feb 17 – Mar 3
Peppers, eggplantTransplant · May 12 – May 26
BeansSow outdoors · May 5 – May 19
Sweet cornSow outdoors · May 5 – May 19
Cucumbers, squashSow outdoors · May 5 – May 19
Fall lettuce, spinachSow outdoors · Sep 20 – Oct 4
GarlicPlant cloves · Nov 1 – Dec 6
Start seeds indoorsSow or plant outdoorsFall plantingMedian last & first frost

Counted from the median last spring frost (Apr 28) and first fall frost (Nov 15) 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 91023 came on Jan 19 — 11 days earlier than 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 Mount Wilson, CA

The soil mapped at the center of ZIP 91023 is Green Bluff family — gravelly sandy loam, well drained.

TextureGravelly sandy loamdrains and warms fast; dries out and needs more feeding
pH6.2slightly acidic; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter1%low — add compost
Hydrologic groupBwater soaks in moderately
Farmland classNot prime farmlandUSDA classification

Map unit: Green Bluff-Hohmann families-Xerorthents complex, 15 to 60 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 91023; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Mount Wilson, CA

Summer high85 °Faverage high, July
Winter low37 °Faverage low, February
Hot days16days a year at 90 °F or more
Freezing nights50nights a year at 32 °F or less
Growing degree days3,992base 50 °F — the season’s usable heat
Longest day14 h 26 mindaylight on June 21
Shortest day9 h 52 mindaylight on December 21
Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully
Precipitation32.2 inper year, rain and melted snow

Average high and low, by month

30°40°50°60°70°80°90°100°32 °FJanuary: average high 56 °F, low 38 °F 56°38° JanFebruary: average high 55 °F, low 37 °F 55°37° FebMarch: average high 59 °F, low 39 °F 59°39° MarApril: average high 63 °F, low 42 °F 63°42° AprMay: average high 70 °F, low 48 °F 70°48° MayJune: average high 79 °F, low 58 °F 79°58° JunJuly: average high 85 °F, low 64 °F 85°64° JulAugust: average high 85 °F, low 64 °F 85°64° AugSeptember: average high 81 °F, low 59 °F 81°59° SepOctober: average high 72 °F, low 51 °F 72°51° OctNovember: average high 63 °F, low 43 °F 63°43° NovDecember: average high 55 °F, low 37 °F 55°37° Dec

Precipitation by month inches

0″2″4″6″8″January: 7.73 in precipitation 7.7 JanFebruary: 8.26 in precipitation 8.3 FebMarch: 5.11 in precipitation 5.1 MarApril: 2.05 in precipitation 2.0 AprMay: 1.05 in precipitation 1.1 MayJune: 0.21 in precipitation 0.2 JunJuly: 0.21 in precipitation 0.2 JulAugust: 0.01 in precipitation 0.0 AugSeptember: 0.32 in precipitation 0.3 SepOctober: 1.31 in precipitation 1.3 OctNovember: 1.88 in precipitation 1.9 NovDecember: 4.03 in precipitation 4.0 Dec

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

Nearby ZIP codes

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

9102410a
Sierra Madre, CA · 4 mi
9100310b
Altadena, CA · 4 mi
9102510a
Sierra Madre, CA · 5 mi
9100110b
Altadena, CA · 5 mi
9110710a
Pasadena, CA · 5 mi
9110410a
Pasadena, CA · 5 mi
9106610a
Arcadia, CA · 6 mi
9107710a
Arcadia, CA · 6 mi

California planting zones →

Questions about 91023

What planting zone is ZIP code 91023?

ZIP code 91023 (Mount Wilson, CA) is in USDA hardiness zone 9a on the 2023 map: the coldest night of an average winter reaches 20 to 25 °F (−6.7 to −3.9 °C).

When is the last frost in Mount Wilson, CA?

The median last spring frost (32 °F) near 91023 is April 28; after May 27 there is only a 10% chance of frost. Data: NOAA 1991–2020 normals, Mt Wilson Cbs station.

When is the first frost in Mount Wilson, CA?

The median first fall frost near 91023 is November 15, giving a median frost-free growing season of 199 days.

When should I plant tomatoes in 91023?

Start tomato seeds indoors around Mar 3 – Mar 17 and set transplants outside around May 5 – May 19, one to three weeks after the median last frost.

How much rain and snow does 91023 get?

About 32.2 inches of precipitation a year (NOAA 1991–2020 normals, Mt Wilson Cbs station).

How hot do summers get in Mount Wilson, CA?

The average high peaks at 85 °F in July, with about 16 days a year at 90 °F or more.

What kind of soil does 91023 have?

The USDA soil survey maps Green Bluff family at the center of ZIP 91023: gravelly sandy loam, well drained, pH about 6.2 (slightly acidic).

Did spring come early in Mount Wilson, CA in 2026?

Spring leaf-out near 91023 came on Jan 19 in 2026, 11 days earlier than the 1991–2020 average (Jan 30), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Mount Wilson, CA?

Intermediate onions: the longest day near 91023 is 14 h 26 min, so day-neutral types bulb best; long-day varieties may not bulb fully.

Did the zone for 91023 change?

Yes. The 2012 USDA map put 91023 in zone 8b; the 2023 map puts it in zone 9a. The newer map uses 1991–2020 data and more weather stations.