Frost Date Calculator.
Frost Date Calculator
Estimate average last spring and first fall frost dates by USDA hardiness zone or by reference city. The first input every garden plan needs.
What "frost date" actually means
The dates you see published are 50% probability dates. That is — on the published last spring frost date, there's a 50% historical chance frost has already happened, and a 50% chance it's still coming.
For tender crops (tomato, basil, pepper), planting on the 50% date will work about half the time. The widely-used safer practice:
- Use the 90% probability date for tender crops (typically 1-2 weeks after the 50% date)
- Use the 10% probability date as the earliest possible — for crops that can tolerate a touch of frost (peas, brassicas, lettuce)
- Always have row cover ready for the first 1-2 weeks; freak frost happens in every region
Why frost dates vary so much locally
| Factor | Effect |
|---|---|
| Elevation | Each 1,000 ft of elevation = ~3-5°F cooler; ~1-2 weeks later spring frost |
| Slope aspect | South-facing slopes warm faster in spring; north slopes lag 1-2 weeks |
| Topography | Cold air pools in low spots; valley bottoms see frost when hilltops 100m higher don't |
| Water proximity | Large lakes / oceans moderate temperature; coastal sites have shorter freeze season |
| Urban heat island | City centers can run 5-10°F warmer than surrounding rural; gardens in sheltered courtyards do too |
| Wind exposure | Sheltered sites trap heat; exposed sites lose it |
The estimates this widget provides are zone-level. For garden-level precision, use a local weather station's 30-year climatology (NOAA NCEI) or talk to neighbors who garden.
Effects of climate change
Frost-free season has been lengthening across most of North America. USDA's 2023 hardiness zone update shifted ~half the country one zone warmer compared to the 2012 map. Practical implications:
- Last spring frost dates trending ~7 days earlier across most of US
- First fall frost trending ~10 days later
- Net effect: ~2-3 weeks longer growing season vs 30 years ago
- However: variability has increased — late-spring freeze events after warm spells are more common, not less
Free under CC BY 4.0. Cite as "OAT Frost Date Calculator (openagriculturetechnology.com)". Reference data from NOAA NCEI 1991-2020 climate normals.