What your USDA zone does and doesn't tell you

USDA builds the map from the average annual extreme minimum winter temperature. In plain words: take the single coldest night of each winter, average those nights over many years, and you have the number. The map shows it in 10°F zones and 5°F half zones, marked a and b.

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026.

The 2023 map uses the years 1991 to 2020, the same period climatologists use for U.S. climate normals, and data from 13,625 weather stations.

Source: USDA ARS, how the 2023 map was made, checked October 6, 2026.

• 10°Fper whole zoneUSDA Plant Hardiness Zone Map (2023), October 6, 2026
• 5°Fper half zone, a or bUSDA Plant Hardiness Zone Map (2023), October 6, 2026
• 13,625weather stations behind the 2023 mapUSDA ARS, how the 2023 map was made, October 6, 2026
• 30 yearsof winters, 1991 to 2020USDA ARS, how the 2023 map was made, October 6, 2026

• It leaves out summer heat. USDA notes that plants grow best within a range of temperatures, both cold and hot. Heat has its own map: the American Horticultural Society's Plant Heat Zone Map counts the days a year that reach 86°F or more.
• It has no frost dates. The map is built from winter lows only, so it doesn't say when to sow or plant out.
• It doesn't say how long the cold lasts, and USDA points out that many plants that survive a short cold snap can't handle longer spells of cold.
• It ignores snow, wind and dry soil. USDA lists wind, soil type, soil moisture, humidity, snow and winter sunshine as things that can greatly affect whether a plant survives.
• It can't see your own yard. The 2023 map is drawn on a grid of half-mile squares, and USDA says there can still be microclimates too small to show. A warm wall or a frost pocket can run warmer or colder than the listing.

Source: USDA ARS, how to use the zone maps, checked October 6, 2026; Iowa State Extension, heat zone map, checked October 6, 2026.

1. 1

   Look up your zone by zip code

   Type your five-digit zip into the zone finder. It returns the zone and the 5°F band behind it, from the 2023 zip-code listing.

   Source: PRISM 2023 USDA hardiness zone GIS datasets and zip-code listing, checked October 6, 2026.

2. 2

   Read the zone range on the tag

   Tags list whole zones, such as zones 4 to 8. That means the plant should handle winters from zone 4 up to zone 8.

3. 3

   Compare the whole number

   Drop the letter from your zone. Zone 6b is 6, and 6 sits inside 4 to 8. If your number is below the range, winter is colder than the plant likes; above it, warmer.

4. 4

   Check what the zone leaves out

   Look at summer heat, how wet the spot stays in winter, wind, and whether the spot is sheltered. These can make a plant fail inside its zone range.

5. 5

   Keep a log

   Write down what you planted and how it came through each winter on the printable hardiness log. After a few winters, the log shows how your own yard behaves.

When USDA released the 2023 map, it said about half of the country shifted to the next warmer half zone, while the rest stayed where it was. USDA's map notes put the overall change at about a quarter zone warmer than the 2012 map across much of the country.

Source: USDA ARS news release, November 15, 2023, checked October 6, 2026; USDA ARS, how the 2023 map was made, checked October 6, 2026.

A half zone is 5°F, so a move from 6a to 6b means the average coldest night now falls in the -5°F to 0°F band instead of -10°F to -5°F. If you picked plants with a 2012 zone in mind, look your zip code up again.

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026.

Each zone starts 10°F above the one before; the b half starts 5°F above the a half.

Source: USDA Plant Hardiness Zone Map (2023), checked October 6, 2026.