What Is a Frost Date? Averages, Probabilities, and the Date to Actually Use

Last frost and first frost

Your last frost date is the average date of the final freezing night in spring. Your first frost date is the average date of the first freezing night in fall. The days between them are your frost-free growing season.

This site uses NOAA's 1991-2020 U.S. Climate Normals, which summarize 30 years of daily temperature records at thousands of weather stations into frost probabilities for each station.

The average date is a coin flip

An average last frost date is the 50% point: in half of years the last frost comes earlier, and in half it comes later. Planting tender seedlings on that exact date means accepting roughly even odds of a frost afterward.

That is why each ZIP code page also shows the 10% and 90% dates. In spring, the 10% date is the date after which frost happens in only about 1 year in 10. It is a safer target for tomatoes, peppers, basil, and other frost-tender plants. In fall, the 10% date is the date by which frost has already arrived in about 1 year in 10, so it marks when to start watching forecasts closely.

Three temperatures, three kinds of frost

Frost dates are reported for several temperatures. At 36°F, frost can form on plants on clear, calm nights even though the air a few feet up is above freezing; this light frost damages the most tender plants. At 32°F the air itself freezes and most warm-season crops are injured. At 28°F, a hard freeze, most annual plants are killed, while hardy crops such as kale and collards survive and can even taste sweeter.

Why your yard may differ

Weather stations measure conditions at one spot. Cold air drains downhill and pools in valleys and low fields, so low spots often frost earlier in fall and later in spring than a station on higher ground or in a town. Cities, large lakes, and coastlines stay warmer at night. Each ZIP code page names the station used and its elevation so you can judge how well it matches your garden.