What your last frost date really means
Every gardening calendar starts from your "last frost date." But that date isn't a deadline that frost respects. It's a probability, and the typical date, the one planting guides count from, is a coin toss.
Where frost dates come from
The dates on this site come from the National Oceanic and Atmospheric Administration (NOAA). Every ten years NOAA publishes "climate normals," 30-year summaries of the weather at thousands of stations. The current set covers 1991 to 2020.
For frost, NOAA starts from each station's nightly low temperatures. It fills gaps in a station's record using its five nearest neighbors. Then it builds 10,000 simulated years by reshuffling the real ones, and counts how often the last freezing night of spring, and the first of fall, lands on each date. NOAA used the same method for the 1981 to 2010 normals and carried it forward.
The result isn't one date. It's a set of dates, each with a probability attached.
The three dates on every page here
For spring, we show three dates from that set:
- The early year. In only 1 spring in 10 is the frost already over by this date. In the other 9, there's still a frost after it.
- The typical year. Half of springs have their last frost before this date and half after. Planting guides count from an "average" last frost date; this is the date we use for it.
- The late year. Only 1 spring in 10 has a frost after this date.
Here's how those three dates look in a few places with pages on this site:
| Place | Early year (1 in 10) | Typical (1 in 2) | Late year (1 in 10) | Spread |
|---|---|---|---|---|
| Atlanta, GA | March 2 | March 24 | April 10 | 39 days |
| Dallas, TX | February 10 | March 2 | March 19 | 37 days |
| Des Moines, IA | April 4 | April 18 | May 2 | 28 days |
| Denver, CO | April 20 | May 4 | May 16 | 26 days |
| Boston, MA | March 25 | April 4 | April 17 | 23 days |
| Minneapolis, MN | April 8 | April 23 | May 7 | 29 days |
| Seattle, WA | February 13 | March 8 | March 29 | 44 days |
| Fairbanks, AK | May 2 | May 13 | May 26 | 24 days |
At the typical NOAA station, the early-year and late-year dates are 31 days apart. So in 8 springs out of 10, the last frost lands somewhere in a window of 31 days. Fall works the same way: in 8 years out of 10, the first frost of fall lands in a window of 28 days.
Why the typical date isn't a safe planting date
Read the middle date again: half of springs have a frost after it. If you set out tomatoes on your typical last frost date and leave them uncovered, you can expect frost to catch them in about half of all years.
That's why the planting rules on this site treat the typical date as a reference point, not a green light. Colorado State University Extension's planting guide sorts vegetables by how much cold they can take:
- Hardy crops such as peas, spinach, lettuce and radishes can go in as early as 2 to 4 weeks before the typical last frost. They may survive a frosty night.
- Semi-hardy crops such as beets, carrots and potatoes can go in up to 2 weeks before it.
- Tender crops such as beans, sweet corn and cucumbers are sown from seed around the typical date.
- Very tender crops such as tomatoes, peppers, eggplant and melons wait until 2 weeks or more after it.
Two weeks after the typical date is close to the late-year date in most places: at the typical NOAA station, the late-year date comes 16 days after the typical one. If you want more insurance than that, every place page here tells you how many days to shift your spring dates to plan around the late-year date instead.
What "frost" means here
NOAA's frost dates count nights when the station's thermometer reads 32°F or colder. But plants can frost over when the thermometer reads a few degrees above freezing.
The reason is where the thermometer sits. The National Weather Service office in Tallahassee explains that air temperature is measured about 2 meters (6.56 feet) above the ground. On clear, still nights, heat radiates away from the ground, so the air right at plant level is the coldest. The official reading can be 33 to 36°F while frost forms on your lettuce.
That's why the National Weather Service issues a Frost Advisory when temperatures are expected to drop to between 33 and 36°F, and a Freeze Warning when it's confident they'll fall below 32°F. NOAA also publishes dates for 36°F, and our pages show them as "frost possible." At most stations they come 9 to 17 days later in spring than the 32°F dates, so check yours.
At the other end, our pages show the typical dates for 28°F, a harder freeze than the 32°F frost line. The National Weather Service's own "hard freeze" threshold varies from office to office.
When frost doesn't come every year
In the warmest parts of the country, some winters pass without a single freezing night. For those stations, NOAA gives frost dates only for the years that have a frost. Our pages say so, and tell you how often frost comes at all. Where it comes in fewer than half of years, we don't build a planting calendar around it, because in most years there's no last frost to plant after.
What a frost date can't tell you
A frost date describes a weather station. Your garden may be warmer or colder. The USDA, describing its hardiness zone map, points out that small heat islands such as blacktop and concrete, or cool spots such as a low place where cold air pools first, can make your yard differ from the area around it. Our guide to why frost dates differ over short distances explains more.
A frost date also can't tell you about this year. It's a summary of 30 years. Use it to plan when to start seeds and order transplants. Then, as planting day comes near, watch the forecast. A Frost Advisory or Freeze Warning tonight outranks any average. If one comes after you've planted, our guide to protecting plants from frost will help.
In short
- Plan with the typical date. It's what seed packets and planting charts count from.
- Plant hardy crops before it, and tender crops well after it.
- Wait for the late-year date, or be ready to cover your plants, before setting out tomatoes and peppers.
- Trust tonight's forecast over any average.
To see your own three dates, look up your ZIP code.
Sources
- U.S. Climate Normals, NOAA National Centers for Environmental Information. Checked October 25, 2026.
- Normals Calculation Methodology 2020, NOAA National Centers for Environmental Information. Checked October 25, 2026.
- Readme: By-Variable and By-Station Normals Files, NOAA National Centers for Environmental Information. Checked October 25, 2026.
- Frost and Freeze Information for the NWS Tallahassee Area, National Weather Service, Tallahassee. Checked October 25, 2026.
- Spring Frost/Freeze Information, National Weather Service, Aberdeen, South Dakota. Checked October 25, 2026.
- Vegetable Planting Guide (CMG GardenNotes #720), Colorado State University Extension. Checked October 25, 2026.
- How to Use the Maps, USDA Agricultural Research Service. Checked October 25, 2026.
More guides: Hardiness zones, and what changed on the 2023 map · How to protect plants from a late frost · Starting seeds indoors: when and how · What to plant, month by month