Arizona · Maricopa County
Why the zone map misleads in the Arizona desert
A USDA hardiness zone is the average coldest night of the year, and nothing else. In Maricopa County the zone is 9a (2 ZIP codes), 9b (100 ZIP codes), 10a (33 ZIP codes) — 135 ZIP codes in all. Plenty of places far from any desert share those zones.
Same zone, different summer
Every ZIP code in the country in these zones, split by whether the NOAA station it cites is low desert by this site's measured test — a summer high of at least 100 °F, no more than 12 inches of rain a year, and a median last spring freeze by or none at all. Medians across the ZIP codes in each group.
| Zone | Group | ZIP codes | Summer high | Rain per year | Mostly in |
|---|---|---|---|---|---|
| 9a | Low desert | 64 | 103.6 °F | 4.78" | Nevada (29), Arizona (25), California (9), Texas (1) |
| Everywhere else | 1,703 | 91.2 °F | 50.94" | Texas (448), Louisiana (275), Florida (213), California (194) | |
| 9b | Low desert | 154 | 103.5 °F | 8.18" | Arizona (116), Nevada (30), California (8) |
| Everywhere else | 1,320 | 91.2 °F | 39.85" | California (626), Texas (301), Florida (238), Louisiana (106) | |
| 10a | Low desert | 73 | 105.2 °F | 5.11" | Arizona (40), California (33) |
| Everywhere else | 719 | 90.2 °F | 26.12" | California (356), Florida (282), Texas (73), Louisiana (6) |
Summer high is the NOAA 1991–2020 mean daily maximum over the three summer months; rain is the annual normal. Within each zone the winter is the same by definition; the summer is not.
Two planting seasons, not one
A zone map implies one spring and one fall. The University of Arizona's calendar for Maricopa County gives tomatoes 2 separate windows: and . And at 1 of the 23 NOAA stations Maricopa County ZIP codes cite, the freeze-free season runs across New Year — the "last spring freeze" there is a midwinter date. No zone number can carry either fact.
What the zone does tell you
It tells you what lives through the winter, and it answers that well — including the plants that fail for want of cold. What survives my winter? runs on it. For the general case, see your hardiness zone is not a planting date.
Where these dates come from
Every planting window on this page is from the University of Arizona Cooperative Extension's Vegetable Planting Calendar for Maricopa County (az1005), by Kai Umeda, revised September 2018, peer reviewed. It lists 53 crops in 24 half-month planting windows; S means seeds, T transplants, X sets or cloves.
Publication: https://extension.arizona.edu/publication/vegetable-planting-calendar-maricopa-county
PDF: https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1005-2018.pdf
⚠️ It is written for Maricopa County, which holds 135 of the 164 ZIP codes in the Phoenix-Mesa-Chandler, AZ metro area. It is not written for the other 29 — Pinal County (29) — nor for Tucson, Yuma or anywhere else in Arizona, and this site does not stretch it to cover them. Freeze dates, summer highs and rainfall are NOAA 1991–2020 normals from the stations named on this page.