Understanding USDA Zones and Frost Dates

By GardenPlanner Team · July 17, 2026

A USDA hardiness zone is based on one thing: the average coldest winter temperature in your area. Zone 3 sees brutal winters (down to -40°F); zone 10 rarely freezes at all. It’s a genuinely useful shorthand, which is why our planting calendar and Garden Designer both use it — but it’s worth understanding what it doesn’t tell you.

What a zone number gets you

For most of the country, “zone” is a reasonable stand-in for “roughly when does it stop and start frosting,” and that’s the number our calendar uses to work out when to start seeds and when it’s safe to move plants outside.

Where the shorthand breaks down

Some regions don’t have a simple single-frost-date growing season at all. The low desert Southwest (Phoenix, Tucson — zone 9b/10a) is the clearest example: instead of one planting season, it has an inverted dual season, with a spring window before the summer heat and a second “monsoon” window in late summer once temperatures ease. Planting tomatoes on a generic zone-10 frost date would put them in the ground right as they’re about to get cooked.

We handle this by using explicit research-based planting windows for zone 10, sourced from Growing In The Garden (a Phoenix-area horticulturist’s practical guide to low-desert timing) and cross-checked against the University of Arizona Cooperative Extension’s official calendar, instead of computing a single frost date. If you’re in zone 10, our calendar and designer will show you both windows, with the exact source cited on the page.

The takeaway

Your zone tells you the broad strokes. For anything more specific — microclimates, unusual regional seasons, local pest pressure — a regional university extension office knows your area better than any national chart, ours included.