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Regional Calendars, Climate & DataFirst and Last Frost Date Maps
A frost date map shows an average, but your crop lives through individual years. Learning to read the probability behind the date lets you choose how much risk to take.
Key points
- A frost date is an average, so half of years have a freeze after the 50 percent date.
- Choose the 10, 50 or 90 percent level based on crop value and your tolerance for loss.
- Use the threshold that matches the crop: 36, 32 or 28 degrees F and colder.
- Check the forecast and soil temperature before planting, whatever the map says.
What does a frost date on a map really mean?
Most frost maps and tables show a typical date for the last spring or first fall freeze, drawn from decades of station records. NOAA's freeze and frost tables from NCEI use three temperature thresholds: 36 degrees F for frost, 32 for freeze and 28 for hard freeze, and the version discussed by NC State Extension is based on 1981 to 2010 normals.
A map date is not a forecast. In about half of all years, a freeze comes after the 50 percent date. That is why seasoned growers do not treat the mapped date as a safe date to plant tender crops.
How do the 10, 50 and 90 percent dates work?
The tables list dates at several probability levels. NC State Extension explains that a 10 percent probability means the event occurs about one year in ten, and 90 percent means about nine years in ten. For a last spring freeze, the 90 percent date is early and high risk, because a freeze after it is still common. The 10 percent date is later and conservative.
The National Weather Service office in Medford, Oregon, gives a worked example for planting frost-sensitive vegetables near the Medford airport on its frost and freeze page. It shows how the 90, 50 and 10 percent dates for frost spread across several weeks, and it says that a grower with no tolerance for frost should wait for the latest date. Use the same logic with your own station.
- Early date, higher risk: fine for hardy crops or crops you can cover.
- Middle date: moderate risk, a common starting point.
- Late date, low risk: choose for expensive transplants or tender crops.
Which temperature threshold should you use?
Match the threshold to the crop. Tender warm-season crops can be hurt near 31 to 33 degrees F, and some are bothered by 36 degree nights in combination with wind or cold soil. Hardy greens and brassicas can handle much colder nights, so a 28 degree table or colder is more relevant for them.
Many states also publish their own freeze probability pages. Nebraska's CropWatch, for example, has dates and probabilities for the last spring freeze across Nebraska, and Kansas State Extension has written about how likely a late spring freeze is in Kansas.
Keep in mind that the thresholds are not interchangeable. A 36 degree table will tell you about the first light frost on tender leaves, while a 28 degree table tells you about the first hard freeze that ends the season for hardy crops. Choose the table that matches the crop and the kind of damage you want to avoid.
How do you find the right station for your farm?
Look for the NOAA station nearest your land that is similar in elevation and exposure. Airports often sit in open, well-drained areas, so a creek-bottom field may run colder on calm, clear nights. If you have your own thermometer records, compare a few spring mornings with the station and note the typical difference.
The Midwestern Regional Climate Center freeze date tool covers counties in the north-central and northeastern states and averages across each county, so local conditions may differ.
Why do frost dates keep changing?
Normals are recalculated, and newer 30-year periods may shift averages slightly. NCEI currently recommends the 1991 to 2020 normals unless you need an older set. Year-to-year weather swings are much larger than the shift between normals, so a late freeze can still arrive well after a mapped date.
Treat any map as a starting point and update your own notes each year.
What is a sensible planting rule based on frost dates?
Pick a risk level for each crop, find that date for your station, and then check the 7 to 10 day forecast and soil temperature before you commit. Keep covers handy for plantings that go in early. If the forecast shows a cold night after planting, delay transplanting or protect the planting.
A map tells you when to start paying attention. The forecast and your own field tell you when to act.
How do you record your own frost dates?
Keep a simple log with the date of the last spring frost and first fall frost you observe in your fields, the low temperature you measured and the crops affected. Over a few years you will see how your land compares with the mapped dates, and you can adjust your chosen risk level with real evidence behind it.
Include notes on the frosts that did damage and the ones that did not. A 34 degree night with wind and a 34 degree night with clear, still air can behave very differently on the same crop.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.