Data: 2023 USDA zones (PRISM/OSU) · NOAA 1991–2020 normals · updated Oct 29, 2026

Planting zone for Walnut, CA

Walnut, CA is in USDA zone 10a across its 2 ZIP codes.

Walnut on the zone map

Outlined: its 1 ZIP code areas · 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

ZIP codes2

Last frostrare

First frostrare

Growing seasonall year

10a 2 ZIPs

Frost forecast — next 7 nights

live from the National Weather Service

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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, 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 91788 came on Jan 17 — 4 days earlier than the 1991–2020 average (Jan 21).

average
Jan 212026
Jan 17

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 Walnut, CA

The soil mapped at the center of ZIP 91788 is Ballona — loam, well drained. Much of the area is built over (urban land); this is the natural soil underneath.

TextureLoama good balance for most vegetables

pH7.8slightly alkaline; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter2%moderate

Hydrologic groupCwater soaks in slowly

Farmland classPrime farmland if irrigatedUSDA classification

Map unit: Urban land-Ballona-Typic Xerorthents, fine substratum complex, 0 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 91788; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Walnut, 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 25 mindaylight on June 21

Shortest day9 h 53 mindaylight on December 21

Onions to growIntermediateday-neutral types bulb best; long-day varieties may not bulb fully

Precipitation16.3 inper year, rain and melted snow

Average high and low, by month

Precipitation by month inches

NOAA NCEI U.S. Climate Normals 1991–2020 — temperature: Pomona/Fairplex, CA (7 mi away); precipitation: Diamond Bar 1.9 Nne, CA (4 mi away). Daylight is calculated from latitude.