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

Planting zone for 91740 — Glendora, CA

ZIP code 91740 (Glendora, Los Angeles County) is in USDA zone 10a: the coldest winter night averages 30 to 35 °F.

Where ZIP 91740 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.

Zone10a30 to 35 °F
Coldest night−1.1 to 1.7 °C
Last frostrare
First frostrare
2012 zone10a

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 10a means →

Frost forecast — next 7 nights

live from the National Weather Service

Loading the forecast for Glendora, 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

A 32 °F frost doesn't happen here in most winters. At the nearest station (NOAA station Pomona/Fairplex, CA — 6 miles away, elevation 1,001 ft) the median growing season is 354 days. When frost does come, it is a midwinter night or two; cover tender plants when a freeze is forecast.

Frost data: NOAA NCEI U.S. Climate Normals 1991–2020.

When spring arrived in 2026

In 2026, spring leaf-out near 91740 came on Jan 17 — 4 days earlier than the 1991–2020 average (Jan 21).

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 Glendora, CA

The soil mapped at the center of ZIP 91740 is Zaca — clay, well drained.

TextureClayholds water and nutrients; slow to drain and warm up
pH7.8slightly alkaline; most vegetables like 6.0–7.0
DrainageWell drainednatural drainage class
Organic matter2.5%moderate
Hydrologic groupCwater soaks in slowly
Farmland classNot prime farmlandUSDA classification

Map unit: Zaca-Apollo, warm complex, 20 to 55 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 91740; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Glendora, CA

Summer high92 °Faverage high, August
Winter low42 °Faverage low, December
Hot days71days a year at 90 °F or more
Freezing nights3nights a year at 32 °F or less
Growing degree days5,557base 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
Precipitation21.3 inper year, rain and melted snow

Average high and low, by month

30°40°50°60°70°80°90°100°32 °FJanuary: average high 68 °F, low 43 °F 68°43° JanFebruary: average high 67 °F, low 45 °F 67°45° FebMarch: average high 72 °F, low 47 °F 72°47° MarApril: average high 76 °F, low 50 °F 76°50° AprMay: average high 78 °F, low 54 °F 78°54° MayJune: average high 84 °F, low 57 °F 84°57° JunJuly: average high 90 °F, low 62 °F 90°62° JulAugust: average high 92 °F, low 63 °F 92°63° AugSeptember: average high 89 °F, low 60 °F 89°60° SepOctober: average high 81 °F, low 54 °F 81°54° OctNovember: average high 74 °F, low 47 °F 74°47° NovDecember: average high 67 °F, low 42 °F 67°42° Dec

Precipitation by month inches

0″1″2″3″4″5″6″January: 4.99 in precipitation 5.0 JanFebruary: 5.48 in precipitation 5.5 FebMarch: 3.13 in precipitation 3.1 MarApril: 1.04 in precipitation 1.0 AprMay: 0.74 in precipitation 0.7 MayJune: 0.22 in precipitation 0.2 JunJuly: 0.08 in precipitation 0.1 JulAugust: 0.00 in precipitation 0.0 AugSeptember: 0.16 in precipitation 0.2 SepOctober: 0.99 in precipitation 1.0 OctNovember: 1.21 in precipitation 1.2 NovDecember: 3.27 in precipitation 3.3 Dec

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Pomona/Fairplex, CA (6 mi away); precipitation: Glendora Fc 287b, CA (2 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

All 8 nearest ZIP codes share zone 10a.

9177310a
San Dimas, CA · 3 mi
9172410a
Covina, CA · 3 mi
9172310a
Covina, CA · 3 mi
9174110a
Glendora, CA · 3 mi
9172210a
Covina, CA · 3 mi
9179110a
West Covina, CA · 5 mi
9176810a
Pomona, CA · 5 mi
9175010a
La Verne, CA · 5 mi

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

Questions about 91740

What planting zone is ZIP code 91740?

ZIP code 91740 (Glendora, CA) is in USDA hardiness zone 10a on the 2023 map: the coldest night of an average winter reaches 30 to 35 °F (−1.1 to 1.7 °C).

How much rain and snow does 91740 get?

About 21.3 inches of precipitation a year (NOAA 1991–2020 normals, Glendora Fc 287b station).

How hot do summers get in Glendora, CA?

The average high peaks at 92 °F in August, with about 71 days a year at 90 °F or more.

What kind of soil does 91740 have?

The USDA soil survey maps Zaca at the center of ZIP 91740: clay, well drained, pH about 7.8 (slightly alkaline).

Did spring come early in Glendora, CA in 2026?

Spring leaf-out near 91740 came on Jan 17 in 2026, 4 days earlier than the 1991–2020 average (Jan 21), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Glendora, CA?

Intermediate onions: the longest day near 91740 is 14 h 26 min, so day-neutral types bulb best; long-day varieties may not bulb fully.