Frost dates: where to find them, how to read them

The official source: NOAA Climate Normals

Frost and freeze dates in the United States ultimately come from NOAA's National Centers for Environmental Information (NCEI). NCEI's U.S. Climate Normals describe typical conditions at thousands of locations, drawn from almost 15,000 weather stations. The official normals use a uniform 30-year window and are regenerated every 10 years; the current set covers 1991–2020.

Alongside the familiar temperature and precipitation averages, NCEI publishes agricultural normals. According to NCEI's article When to Expect Your Last Spring Freeze, these include the chance of freezing temperatures on a given day, other cold thresholds, growing season length, first fall freeze dates and growing degree days, all by climate station.

Four ways to look up your dates

Pick the station closest to your garden in both distance and elevation. Climate.gov notes that the last freeze generally arrives later farther north and at higher elevations, and that gray squares on its map mark places where no date could be calculated.

What "average" really means

The word "average" hides a probability. NC State Extension's guide to NCEI freeze tables explains that each date carries a probability of a freeze occurring on or after it (in spring) or on or before it (in fall). The 50% date is the median. The 10% level corresponds to roughly 1 year in 10, and the 90% level to roughly 9 years in 10.

NC State's Raleigh example makes this concrete for spring frost: 90% on March 28, 50% on April 12, and 10% on April 26. Planting tender crops on March 28 means a later frost is still likely; planting on April 26 means a later frost happens only about one spring in ten.

Frost, freeze and hard freeze

NC State adds that frost depends on humidity as well as temperature, so a freeze can occur without visible frost and frost can form without a freeze. Most planting guides, including the ones behind this site's calendar, simply say "last frost".

Choosing a date for your calendar

• Frost-tender crops (tomatoes, peppers, eggplant, melons, basil): Colorado State lists most as "very tender". A conservative 10% spring date, or waiting until after the 50% date as UNH and CSU advise, protects the investment in transplants. NWS Medford calls its 10% spring date the "green date" for planters with no tolerance for frost risk.
• Hardy crops (peas, spinach, cabbage family): these are planted weeks before the last frost anyway, so the 50% date is a reasonable anchor.
• Season length: NC State notes that the logic flips for freeze-free periods: the 90% figure is the conservative one. Montana State suggests counting the days between your two frost dates before choosing long-season crops.

Once you have both dates, enter them in the planting calendar.