How Frost Dates Work: What the Average Really Means

An "average last frost date" is not the day frost stops. In half of all years, frost comes later. Here is how frost dates are calculated and how to use the safe, typical, and risky dates on every FrostCal page.

The average is a coin flip

Seed packets and garden centers talk about "your last frost date" as if it were a fixed point on the calendar. It isn't. A frost date is a statistic: the date by which the last spring frost has happened in a given share of past years.

The number most sources quote is the median, the date that splits past years in half. In half of the years on record, the last frost came before it. In the other half, frost returned after it. Planting tomatoes on your average last frost date is a coin flip.

Where the numbers come from

FrostCal's dates come from NOAA's 1991–2020 U.S. Climate Normals. For each weather station, NOAA looked at 30 years of daily low temperatures and recorded, for every year, the last spring day and the first fall day when the air dropped to 32°F or below. From those 30 pairs of dates it calculates the probability of frost before or after each day of the year.

Each city page uses the nearest station with complete frost data and tells you which station that is and how far away it sits. Most cities are within about 10 km of their station.

Reading safe, typical, and risky dates

Every FrostCal city page shows three dates for each season, drawn from those probabilities.

Last spring frost:

• Risky: in 9 of 10 years, at least one more frost comes after this date. Fine for hardy crops; a gamble for anything tender.
• Typical: the median. Frost comes later in about half of all years.
• Safe: frost comes later only about 1 year in 10. This is the date to plan tender transplants around if you don't want to watch the forecast.

First fall frost:

• Safe (early): frost arrives before this date about 1 year in 10. Have tender crops harvested or covered by then.
• Typical: the median first fall frost.
• Risky (late): by this date, frost has arrived in 9 of 10 years. Counting on warm weather past it is wishful thinking.

The gap between the safe and risky dates is often three or four weeks. That spread is the real answer to "when is it safe to plant?" It depends on how much risk you're willing to take with each crop.

Why your yard may differ from the station

Weather stations measure air temperature about five to six feet above open, level ground. Your garden is not a weather station, and several things can make it colder or warmer than the official reading:

• Clear, calm nights. Leaves and soil lose heat to the open sky and can drop several degrees below the air temperature. Frost often forms on plants when the official low is 33–36°F.
• Low spots. Cold air is heavy and drains downhill. Valleys, hollows, and the bottom of a slope can run noticeably colder than the surrounding area.
• Cities and water. Pavement and buildings hold heat, so urban gardens often escape frost that hits nearby suburbs. Gardens near large lakes or the ocean also tend to frost later in fall.
• Elevation. Temperature generally drops with height. A garden a few hundred feet above its station can run several days behind it in spring.

Keep a note of when frost actually hits your garden for a few seasons. After two or three years you'll know whether to shift the published dates earlier or later.

How to use frost dates in practice

• Hardy crops like peas, spinach, and kale can go in weeks before the typical last frost. See frost vs. freeze for which crops handle which temperatures.
• Tender crops like tomatoes, peppers, basil, and squash should wait until after the typical date, or the safe date if you won't be watching the weather.
• Seed starting counts backward from the typical last frost. See when to start seeds indoors.
• Fall crops count backward from the first fall frost. See fall garden planting.

Averages plan the season; forecasts make the call

Frost dates are for planning: ordering seed, scheduling seed starting, sketching the season. Within a week or two of planting, the forecast matters more than any average. If your local forecast calls for a low near 36°F on a clear night, treat it as a frost risk and cover tender plants, whatever the calendar says.

The normals are also updated once a decade. The current 1991–2020 set will be replaced by 2001–2030 normals in the 2030s, and dates may shift a few days as they are.