What Is a USDA Hardiness Zone?
The Short Answer
A USDA hardiness zone tells you how cold your winters get. It's a number between 1 (coldest) and 13 (warmest), based on the average annual extreme minimum temperature in your area. It matters because it determines which perennial plants can survive your winters and serves as the starting point for calculating your planting dates.
The Zone System Explained
The USDA Plant Hardiness Zone Map divides North America into 26 zones (13 numbered zones, each split into "a" and "b" halves). Zone 1a, in interior Alaska, sees winter lows below -60°F. Zone 13b, in parts of Hawaii and Puerto Rico, never dips below 65°F.
Each full zone represents a 10°F temperature difference. The "a" and "b" subzones divide that further — "a" is the colder half (5°F colder minimum) and "b" is the warmer half. So Zone 6a (winter lows of -10°F to -5°F) is slightly colder than Zone 6b (-5°F to 0°F).
Why Zones Matter for Planting
Your zone determines two critical things for your garden. First, it tells you which perennial plants (trees, shrubs, perennial flowers, fruit bushes) can survive outdoors year-round without protection. A plant rated for "Zones 5-9" can handle winters in those zones but would likely die in Zone 4 winters.
Second, your zone is strongly correlated with your frost dates — the dates of your last spring frost and first fall frost. These frost dates are the foundation of every planting calendar. When you see "start seeds 6 weeks before last frost," you need to know your frost date, which comes from your zone.
How to Find Your Zone
The easiest way is to enter your zip code on our homepage tool. We'll show your zone along with your frost dates. You can also look it up on the official USDA Plant Hardiness Zone Map.
Zones Aren't Everything
Your zone captures one important variable — winter cold — but it doesn't capture everything that affects your garden. It doesn't account for summer heat, rainfall, humidity, soil type, elevation, or the microclimates in your specific yard. A zone 7 garden in the humid Southeast grows very differently from a zone 7 garden in the arid Pacific Northwest, even though they share the same winter low temperatures.
That's why our plant-by-state pages include state-specific soil and climate information — because zone alone doesn't tell the whole story.
Use our Frost Date Lookup tool to find your zone and frost dates instantly.