Plant hardiness zones

The USDA Plant Hardiness Zone Map tells you roughly how cold your winters get. It’s the standard way US nurseries label which perennial plants, shrubs and trees will survive where you live.

What a zone means

Each zone is based on the average annual extreme minimum temperature: the coldest night of each year, averaged over 30 years (1991 to 2020 for the 2023 map). Zones run from 1 (coldest) to 13 (warmest), each covering 10°F, and each is split into an “a” and “b” half of 5°F. So 7a is the colder half of zone 7 and 7b the warmer half.

It is not the coldest it has ever been, or will be. It’s an average of extremes.

Finding your zone

Enter a ZIP code on the USDA map site, or click a spot on the map. State maps can be downloaded and printed. The underlying data comes from Oregon State University’s PRISM Climate Group and can be downloaded as GIS files or by ZIP code.

Using it well

• A plant at the cold edge of its range can thrive for years, then die in one rare cold snap. Give yourself a margin.
• Zones only measure winter cold. Wind, soil, soil moisture, humidity, snow cover, winter sun and summer heat all matter too.
• Your own land has microclimates. Low spots collect frost, slopes and walls trap warmth, and cities run warmer than the countryside around them. Higher ground is usually colder.
• An early hard freeze in autumn can hurt plants before they’ve hardened off, even if it doesn’t reach the zone’s low.
• If your zone changed on the new map, keep what’s thriving. The shift is partly a newer averaging period and partly better data, and USDA says zone changes aren’t reliable evidence of climate change on their own.

For a Leafhouse project

Your zone shapes what a food forest, orchard or perennial garden can hold, and a greenhouse or south wall can push it a half zone or more. In Phase 3, Leafhouse will look up your zone automatically from your address.