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Regional Calendars, Climate & DataUSDA Zone Changes Over Time
The USDA Plant Hardiness Zone Map was updated in 2023, and about half the country moved to a warmer half-zone. Before you act on that, it helps to know exactly what the map measures.
Key points
- Zones show average annual extreme winter minimums, not frost dates or planting dates.
- The 2023 map uses 1991 to 2020 data and about half the country moved to a warmer half-zone.
- A zone shift is a small change and a harsh winter can still exceed it.
- Use local freeze records for annual crops and zone data only for perennials.
What does the USDA Plant Hardiness Zone Map measure?
The map reports the average annual extreme minimum temperature at a location: the coldest night of the year, averaged across many years. Zones span 10 degrees F and are divided into 5 degree half-zones labeled a and b. It helps judge which perennial plants, trees and shrubs are likely to survive winter in a place.
It does not show the coldest temperature ever recorded or the coldest one that will occur. It also does not give frost dates or planting dates, so it cannot tell you when to plant tomatoes or sow corn.
What changed in the 2023 update?
USDA's Agricultural Research Service and Oregon State University's PRISM Climate Group released the first update since 2012. The new map uses weather station data from 1991 to 2020 and draws on 13,412 stations, up from 7,983. Alaska received much finer detail. About half of the country moved to the next warmer half-zone, and the rest stayed in the same half-zone, though some areas warmed without changing half-zones.
The developers cautioned that these changes should not be read as solid evidence of climate change, because annual extreme lows vary widely and both the mapping methods and the number of stations changed between versions.
Should you change what you grow because your zone moved?
Be careful. A half-zone shift is a small change in the average winter low, and a single severe winter can still exceed it. If you grow perennial crops such as fruit trees, berries or asparagus, consider whether you can accept the loss in a harsh year before pushing a variety to the edge of its range.
Many perennial failures come from other factors: wet winter soils, late-spring frost on early blooms, wind exposure, or lack of dormancy chill. A new zone number does not address any of these.
Another useful habit is to read variety information alongside the zone. Plant tags and extension fruit and ornamental guides often list a hardiness range, and the safest choices are those with a margin below your local average minimum, not right at it.
- Check variety-level hardiness ratings from your extension service.
- Consider bloom timing and late frost risk for fruit crops.
- Test a few plants before committing acreage.
How do zones differ from frost dates?
Zones describe winter cold. Frost dates describe the spring and fall boundaries of the growing season. A warm coastal location and a cool inland location can share a zone and have very different frost dates, and a place can have a long season but a low winter minimum.
For annual vegetables and field crops, frost date probabilities, soil temperature and growing degree days matter much more than the zone.
How can you use the map responsibly?
Use the interactive map to look up your location at the half-zone level, and treat the result as one input for perennial choices. Compare it with plant tags or extension recommendations, and remember that your own site may be warmer or colder than the map shows because of slope, buildings or wind.
Add your own record of winter lows to the picture. A min-max thermometer in a few spots across your property will quickly tell you whether your sites match the map.
Treat the zone as a screening tool. It narrows a long list of perennial crops to the ones worth trying, and then your own site notes and local grower experience decide which ones you actually plant.
What do zone changes mean for crop calendars?
Very little directly. A crop calendar relies on spring and fall freeze probabilities, soil temperature, and heat accumulation. A zone change can hint that winters are milder on average, but it does not move a planting date.
If you want to adjust your calendar, look at freeze date records for your own county across decades and see whether your chosen risk dates have shifted.
Where can you look up the map and the details?
The interactive map is available from the USDA Agricultural Research Service, and it lets you enter a ZIP code or zoom to your location. The site also includes a section of tips for growers, and the data can be downloaded and printed for free.
Check the zone for your actual location rather than a nearby city. Because the update uses far more stations than before, the detail around your farm may differ from what you remember from the older map.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.