Similar to the USDA hardiness zones map which helps growers anticipate the minimum annual temperature in their region, the annual frost dates easily help catalog the first and last 32°F days of the year. These first and last frost days of the year essentially “bookend” the growing season as we know it, helping growers anticipate when it's time to sow and when it’s time to reap. While hardiness zones were created from 30 years of USDA climate data, the average first and last frost dates were also similarly designed from three decades of research by the National Oceanic Atmospheric Administration (NOAA).
Frost dates are specific to small regional climates rather than political boundaries and can fluctuate drastically based on terrain, elevation, and microclimates. The western United States, for example, has much more varied frost dates due to the presence of severe elevation changes while the east coast features more stable and predictable frost dates. Some zip codes experience such volatile, unpredictable weather patterns that the NOAA has classified them as either “Too Warm” or “Too Cold to Compute”.
The spring frost date, often known as final frost, marks the last day in spring when temperatures will be below freezing, causing the soil to warm enough to till and directly sow many early spring favorites such as arugula, lettuce, and beets. Perennial crops will begin to emerge from winter dormancy once conditions warm to above 32°F and continue vegetative growth. For most regions, the final frost date does not correlate with the first day of spring and generally is still not warm enough to plant any summer crops such as peppers, melons, or cucumbers.
The fall frost date, sometimes referred to as first frost, signifies the first day in autumn when temperatures officially cool to 32°F or below, causing the soil to no longer be workable while prompting perennial plants to begin their winter dormancy. The fall frost date is generally the absolute latest that most gardeners can sow winter greens, cover crops, or root vegetables for spring. The first frost date has no correlation to the first day of autumn which, for most in the country, the frost doesn’t arrive until several weeks into the season.