Understanding USDA hardiness zones

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.

The 2023 map update

The USDA released an updated map in November 2023, built from 1991-2020 temperature records instead of 1976-2005. Roughly half the country moved into the next warmer half-zone.

Part of that shift reflects warmer winters and part reflects more weather stations and finer mapping. Either way, a new zone number is not a reason to rush into perennials that were marginal for you before - one hard winter can still kill them.

Your garden has its own microclimate

A garden can run warmer or colder than its zone. South- and west-facing walls, paving and cities hold heat; low spots, valley floors and open, windy sites run colder, because cold air drains downhill and settles.

Snow cover insulates roots, so a snowy Zone 5 winter can be easier on perennials than a bare, freezing Zone 6 one. Gardeners often grow plants half a zone beyond their rating in a sheltered corner - and lose them in a frost pocket the map says should be fine.

Heat and chill matter too

Hardiness is only about cold. Summer heat, humidity and rainfall decide whether many plants thrive, so a plant hardy in Zone 8 may still struggle in a hot, humid Zone 8 summer.

For fruit trees, the number of winter chill hours matters as much as the zone - a variety needs enough cold to flower well. The chill hours calculator helps you match varieties to your area.

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.

Common questions