What a hardiness zone means

The USDA Plant Hardiness Zone Map divides the country into 13 zones, each split into "a" and "b" half-zones of 5 °F. Your zone is the average annual extreme minimum temperature for 1991–2020 — how cold a typical winter's coldest night gets. A perennial labelled "hardy to zone 6" should survive winters in zone 6 or warmer (7, 8, …).

Two honest caveats. First, the zone describes winter lows only — it says nothing about frost dates, summer heat, humidity or rainfall, so it is a starting point, not a full planting calendar. Second, the 2023 edition (the current one, used here) shifted roughly half of the country a half-zone warmer than the old 2012 map, because it uses newer temperature normals and better mapping — if a book or plant tag disagrees with this lookup, it is probably quoting the old map.

The zone comes from the USDA's own ZIP-code tables — the same lookup the official USDA site uses, not read off a map by eye. Each ZIP's page here also shows its monthly climate normals next to the zone. Developers: the zone is in every US record of the free API (hardiness_zone), and AI assistants get it through our MCP server.