Frost dates are the backbone of every planting calendar, but they are probabilities, not promises. Here is what the numbers mean and how to use them well.
Where the numbers come from
NOAA’s National Centers for Environmental Information (NCEI) publishes U.S. Climate Normals every ten years. The current edition covers 1991–2020. For each station with enough daily temperature records, NCEI computes the dates of the last spring and first fall occurrence of 36°F, 32°F and 28°F, and turns those 30 years into probabilities. We use these for 7,125 stations.
Reading the probabilities
- Spring, 50%: half of years have a frost after this date. This is the usual “average last frost”.
- Spring, 10%: only one year in ten has a frost after this date. A safer target for tender transplants.
- Fall, 50%: half of years have their first frost before this date.
- Fall, 10%: only one year in ten has its first frost before this date — a safe end date for tender crops.
The gap between the 10% and 90% dates is often four to six weeks, which is why the same calendar can feel early one year and late the next.
Which temperature threshold?
- 36°F: on clear, calm nights the ground and leaves can be several degrees colder than the thermometer at 5 ft, so frost can form on plants at an official 36°F. Use this for very tender plants such as basil and cucumbers in frost-prone spots.
- 32°F: the standard “frost/freeze” date used by most charts.
- 28°F: a hard freeze that kills most warm-season crops and damages many half-hardy ones. Kale, collards, spinach and garlic usually survive.
Adjusting for your garden
- Elevation: higher usually means later spring frost and earlier fall frost.
- Low spots: cold air flows downhill and pools, so valley floors and the bottoms of slopes frost first.
- Cities and water: urban heat and large lakes or the ocean delay frost.
- Walls and covers: a south-facing wall, row cover or cold frame can buy several degrees.
Why not hardiness zones?
USDA plant hardiness zones describe the average coldest winter night and are used to choose perennials, shrubs and trees. They don’t tell you when to plant tomatoes. We don’t show zones on this site because the official zone map data is owned by Oregon State University under terms that restrict how it can be altered and shown, and we only use openly licensed data.
Questions gardeners ask
Are frost dates getting earlier?
Across much of the U.S. the frost-free season has lengthened over recent decades, which the 1991–2020 normals reflect, but the swing from one year to the next is much larger than the long-term trend.
What should I do if frost is forecast after I plant?
Cover tender plants overnight with frost cloth, buckets or sheets (not plastic touching leaves), water the soil during the day so it holds heat, and uncover in the morning.