Garden Frost Date Calculator

A frost date is the historical average date on which the air temperature reaches 32 F (0 C) at a given location. NOAA's National Centers for Environmental Information calculates these averages from decades of weather station records. There are two common probability thresholds: the 50% probability date (average last frost) and the 10% probability date (later, safer date for tender crops).

NOAA provides climate data including frost dates at the National Centers for Environmental Information (ncdc.noaa.gov). The USDA Plant Hardiness Zone Map (planthardiness.ars.usda.gov) shows hardiness zones which correlate with frost timing. Many state cooperative extension services also publish tables of frost dates by city.

The growing season is the number of frost-free days between the last spring frost and the first fall frost. In northern states such as Minnesota or Maine, the growing season may be only 100 to 130 days. In the Deep South and California, frost-free periods of 250 to 365 days allow nearly year-round production.

A light frost occurs when surface temperatures drop to 28 to 32 F and kills tender plants. A hard freeze (below 28 F) kills most annuals and many perennials. The NOAA definitions are: light freeze = 29 to 32 F; moderate freeze = 25 to 28 F; severe freeze = 24 F and below. Hardy crops (kale, broccoli) can survive light frosts; tender crops (tomatoes, basil) are killed by any frost.

Yes. Row covers (floating row cover, spunbond fabric) protect plants from frosts down to approximately 26 to 28 F, extending the season by 2 to 4 weeks at each end. Cold frames extend by 4 to 6 weeks. Unheated high tunnels (hoop houses) can extend growing seasons by 6 to 10 weeks or allow nearly year-round production in many climates.