Frost Date
The average calendar date of the last spring frost or first fall frost at a given location. Used to determine planting and harvest windows for frost-sensitive crops.
Frost dates are the cornerstone of seasonal planting planning. Two frost dates define your growing season: the last spring frost date (after which you’re generally safe to transplant frost-tender crops) and the first fall frost date (when the season ends for those same crops).
How Frost Dates Are Calculated
USDA frost date maps are based on historical weather station data, showing the date by which there is a 10%, 30%, 50%, 70%, or 90% probability that the last frost of spring has occurred. Most planting guides use the 50% probability date - the date by which there’s a 50/50 chance the last frost has already happened. More conservative planning uses the 10% probability date (90% chance the last frost has passed).
The 50% date is the commonly published “frost date.” In practice, using it for frost-sensitive crops means you accept roughly a 50% chance of a frost event after planting. If you want to reduce that risk to 10%, use the 90% probability date (typically 2-4 weeks later than the 50% date).
Finding Your Dates
The USDA Risk Management Agency publishes frost probability data by weather station at rma.usda.gov. State extension services publish regional frost date maps. Local weather stations and agricultural extension offices have precise historical data for your specific area.
City frost dates often differ significantly from suburban and rural locations in the same metropolitan area due to urban heat island effects.
Using Frost Dates in Planning
Last spring frost date: the target for transplanting tomatoes, peppers, eggplant, basil, and other frost-intolerant crops outdoors. Count backward from this date to determine when to start seeds indoors. Tomatoes at 8-week indoor start: if last frost is May 15, start seeds around March 20.
First fall frost date: count backward from this date using each variety’s days-to-maturity to determine the last safe planting date. A 70-day tomato transplanted after August 6 (in a zone with October 15 first frost) will not have time to ripen fruit before frost.
Frost vs. Freeze
A frost occurs when air temperatures drop to 32°F and ice crystals form on surfaces. Frost damage depends on temperature, duration, plant type, and humidity. Light frost (28-32°F) damages frost-sensitive plants but may not kill established cool-season crops. Hard freeze (24°F and below) kills most annual vegetables.
Frost dates are calibrated to 32°F events. In practice, some tender plants are damaged at temperatures several degrees above 32°F when humidity and wind conditions cause plants to cool below ambient air temperature (radiative cooling). Basil, for instance, shows chill injury at 50°F and below, well before frost temperatures.
Extending Beyond Frost Dates
Row cover (floating row cover, frost fabric) provides 2-5°F of frost protection when draped directly over plants. Cold frames and hoop houses extend the season further. With frost protection tools, you can plant earlier in spring and harvest later into fall than bare frost dates suggest.