Frost dates by ZIP code: your last spring and first fall frost

How to read your frost dates

A frost date is a probability, not a promise. The 50% date is the one most charts call your "average" frost date: in about half of years the last spring frost comes later than it, and in about half of years the first fall frost comes earlier.

• Safer spring date (10%). Only about 1 year in 10 has a frost after this date. It is usually two to three weeks after the 50% date.
• Early fall frost (10%). About 1 year in 10 has its first frost by this date, so crops that frost kills are at risk from here on.
• Hard freeze (28°F). The 50% dates for 28°F, the usual threshold for a hard freeze. Frost-tolerant crops such as cauliflower are usually damaged in the upper 20s; very hardy ones such as kale hold on into the low 20s. See how cold each crop can take.
• Growing season. The days between the two 50% dates.

These dates are for an air temperature of 32°F at the weather station. On clear, calm nights, frost can form on leaves while the air is still a few degrees above freezing. The seasonal planner explains frost, freeze, and hard freeze.

Which date should you plan with?

Match the date to what you could lose. Cheap seed you can sow again can go in around the 50% date. Transplants you raised all winter, or a tree you cannot replace this year, are safer after the 10% date, or set out with frost protection ready.

Use the links under your results to carry the date into the seed starting calculator, which counts back from your last spring frost, and the fall planting calculator, which counts back from your first fall frost.

Why your garden can differ from the station

The results show how far away each station is and how high it sits, because both matter. A station on a hilltop or at a city airport can run warmer than a valley garden a few miles away, and a station in the mountains can run colder than you. Within one yard, low spots frost first, because cold air drains downhill and pools.

When the nearest stations disagree, the truth for your garden is usually somewhere in between. Note the first and last frosts you actually see each year; after a few seasons, your own record is the best guide. The planner shows how to set your own local anchors.

Where the numbers come from

The dates are NOAA's 1991–2020 U.S. Climate Normals, which give the probability dates of the last spring and first fall freeze for each weather station with enough temperature records. 6,948 stations have frost dates. At about 330 more, NOAA could not compute them: in the western mountains and Alaska, freezing nights can happen in any month, and in warm places such as south Florida, coastal California, Hawaii, and Puerto Rico they are too rare. The lookup says so when that is the case.

Each ZIP code is placed at the center point of its ZIP Code Tabulation Area from the Census Bureau, which covers the 50 states, Washington, D.C., and Puerto Rico. The three stations are the nearest in a straight line; the lookup does not adjust for elevation. ZIP codes used only for post office boxes have no area of their own, so if yours is not found, try the ZIP code where your garden is.

Sources

• NOAA National Centers for Environmental Information: U.S. Climate Normals — the 1991–2020 normals, including freeze probabilities by station.
• NOAA NCEI: 1991–2020 annual and seasonal normals documentation — definitions of the 10%, 50%, and 90% freeze dates and the flags for stations without dates.
• U.S. Census Bureau: Gazetteer Files — the center point of each ZIP Code Tabulation Area.