Understanding USDA Hardiness Zones
Updated June 2026 · SowWhen
The USDA Plant Hardiness Zone map divides the United States into zones based on the average annual minimum winter temperature, in 10-degree Fahrenheit bands. Zone 5 averages a coldest night of -20 to -10°F; zone 9 rarely drops below 20°F. Each whole zone is split into an "a" and a "b" half for finer detail, so "7b" is the milder half of zone 7. It is the single most common piece of climate shorthand in American gardening, and it's printed on nearly every plant tag and seed packet.
What the zone tells you
The zone is a measure of winter cold-hardiness: whether a perennial, shrub, or tree can survive your typical winter. That's why nursery tags list a range — a plant rated "zones 5–9" should reliably overwinter anywhere in that band. If you're choosing fruit trees, berry bushes, or perennial herbs, matching the plant's rated range to your zone is the first check to make; a fig that's hardy to zone 7 will struggle to survive an unprotected zone 5 winter.
What the zone does not tell you
This is where beginners get tripped up. The zone says nothing about summer heat, rainfall, humidity, soil, day length, or — crucially for vegetable gardeners — the length of your frost-free growing season. Two towns in the same zone can have very different planting calendars because their frost dates and summers differ. Coastal and inland areas can share a zone yet feel like different climates. So for annual vegetables, which live and die within a single season, your frost dates matter far more than the zone number itself.
How the number and the frost dates work together
Think of it as two tools for two jobs. Use your hardiness zone to choose perennials and to locate your region on the map. Then use your local frost dates — which SowWhen derives from your zone — to time when you actually start seeds, transplant, and sow each annual crop. The zone gets you in the right neighborhood; the frost dates give you the exact address. SowWhen ties them together so you don't have to look up two separate things.
Finding your zone
You can look up your zone by ZIP code — the map is granular enough that neighboring towns sometimes differ by half a zone. The map was last updated by the USDA in 2023 using thirty years of weather data, and a warming climate has nudged many areas half a zone warmer than older maps showed. If you're working from a plant tag or an old gardening book, double-check against the current map. Enter your ZIP in SowWhen and it will pull your zone and build the calendar around it in one step.
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