Understanding USDA hardiness zones
Last reviewed September 2026 · written from public-domain USDA/NOAA data and Cooperative Extension references — see how we source data
The USDA Plant Hardiness Zone Map divides North America into bands based on the average coldest winter temperature. It is the standard shorthand for "will this perennial survive my winter" — and it is regularly misused as a planting calendar, which it is not.
What the number means
Each zone represents a 10°F band of average annual extreme minimum temperature. Zone 6 averages a coldest night of -10 to 0°F; Zone 7 is 0 to 10°F, and so on. The a and b halves split each band into 5°F steps.
It is an average of the coldest night per year over three decades, not the record low and not a typical winter night. A Zone 6 garden will occasionally see colder.
What it is good for
Choosing perennials, shrubs and fruit trees. A plant rated "hardy to Zone 5" should survive winter anywhere in Zone 5 or warmer. This is genuinely useful and worth matching carefully for anything you expect to live for years.
It is also a quick way to compare climates: a Zone 8 gardener and a Zone 5 gardener face very different seasons. Look up your zone by state or ZIP.
What it does not tell you
When your last spring frost is, when your first fall frost is, how long your frost-free season runs, or how hot your summer gets. Two places in the same zone can have last-frost dates a month apart because of elevation, latitude and proximity to water.
For deciding when to sow and transplant annual vegetables, your local frost dates are the right tool. Use the zone as a sanity check, not the schedule.
Using both together
Pick perennials by zone. Plan the vegetable year by frost dates. If you only know your zone, the planting calendar will use a typical frost window for that zone as a starting point, but entering your state or your own dates gives a noticeably better result.