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

Planting zone for 85321 — Ajo, AZ

ZIP code 85321 (Ajo, Pima County) is in USDA zone 9b: the coldest winter night averages 25 to 30 °F.

Where ZIP 85321 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 frostrare

First frostrare

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 Ajo, AZ…

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 Ajo, AZ — 16 miles away, elevation 1,752 ft) the median growing season is 347 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 85321 came on Jan 19 — 5 days earlier than the 1991–2020 average (Jan 24).

average
Jan 242026
Jan 19

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).

Native plants for zone 9b in Arizona

499 plants native to the lower 48 states, recorded in Arizona, and rated by the USDA to survive 25 °F or colder — the bottom of zone 9b. Each links to its USDA PLANTS profile.

Source: USDA, NRCS. 2026. The PLANTS Database (plants.usda.gov), National Plant Data Team — state distribution, native status and the “Temperature, Minimum” characteristic. Check light, moisture and soil needs before buying; local nurseries and your state native plant society can confirm what thrives in Ajo, AZ.

Soil in Ajo, AZ

The soil mapped at the center of ZIP 85321 is Delthorny — extremely stony fine sandy loam, well drained.

TextureExtremely stony fine sandy loamdrains and warms fast; dries out and needs more feeding

pH7.9alkaline; most vegetables like 6.0–7.0

DrainageWell drainednatural drainage class

Organic matter0.75%low — add compost

Hydrologic groupDwater soaks in very slowly

Farmland classNot prime farmlandUSDA classification

Map unit: Delthorny-Garzona-Rock outcrop 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 85321; soils change within a few hundred feet, so a home soil test (pH, nutrients) is still the best guide.

Climate in Ajo, AZ

Summer high104 °Faverage high, July

Winter low43 °Faverage low, December

Hot days168days a year at 90 °F or more

Freezing nights2nights a year at 32 °F or less

Growing degree days8,674base 50 °F — the season’s usable heat

Longest day14 h 17 mindaylight on June 21

Shortest day10 h 01 mindaylight on December 21

Onions to growShort-daysummer days stay under about 14 h 20 min, so long-day varieties won’t bulb

Precipitation7 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: Ajo, AZ (16 mi away); precipitation: Ajo, AZ (16 mi away). Daylight is calculated from latitude.

Nearby ZIP codes

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

853419b

Lukeville, AZ · 28 mi

Gila Bend, AZ · 45 mi

851729a

Stanfield, AZ · 52 mi

851939a

Casa Grande, AZ · 53 mi

851389a

Maricopa, AZ · 57 mi

853229b

Arlington, AZ · 58 mi

Arizona planting zones →

Questions about 85321

What planting zone is ZIP code 85321?

ZIP code 85321 (Ajo, AZ) 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).

How much rain and snow does 85321 get?

About 7 inches of precipitation a year (NOAA 1991–2020 normals, Ajo station).

How hot do summers get in Ajo, AZ?

The average high peaks at 104 °F in July, with about 168 days a year at 90 °F or more.

What kind of soil does 85321 have?

The USDA soil survey maps Delthorny at the center of ZIP 85321: extremely stony fine sandy loam, well drained, pH about 7.9 (alkaline).

Did spring come early in Ajo, AZ in 2026?

Spring leaf-out near 85321 came on Jan 19 in 2026, 5 days earlier than the 1991–2020 average (Jan 24), according to the USA National Phenology Network Spring Index.

What kind of onions grow in Ajo, AZ?

Short-day onions: the longest day near 85321 is 14 h 17 min, so summer days stay under about 14 h 20 min, so long-day varieties won’t bulb.