Frost Dates Explained: The Two Numbers That Run Your Garden
Updated June 2026 · SowWhen
If you learn only two dates as a gardener, learn your average last spring frost and your average first fall frost. Between them lies your growing season — the frost-free stretch in which tender plants can live outdoors. Almost every planting decision is really a decision about where you are relative to those two dates.
What the dates actually mean
The last spring frost is the average date after which your area no longer expects a frost. It is the green light for tender crops like tomatoes, peppers, beans, squash, and basil, which a light frost will kill outright. The first fall frost is the mirror image: the average date in autumn when frost typically returns, ending the season for those same tender crops. Hardy plants — spinach, kale, garlic, peas — ignore both to a degree, which is exactly why they can be planted earlier and harvested later.
SowWhen stores representative frost dates for each USDA hardiness zone and resolves your crop timing against them. That's how a rule like "transplant after last frost" becomes an actual week on your calendar.
Why they're averages — and what that costs you
A frost date is a probability, not a promise. "Average last frost of April 15" typically means there's roughly a 50% chance of a frost after that date in any given year. Half the time, in other words, you'll still get a late frost. That's why experienced gardeners treat the frost date as the center of a window, not a starting gun, and keep an eye on the ten-day forecast around it.
The practical rule: for anything you can't afford to lose, wait a week or two past the average last-frost date, or be ready to cover plants with a row cover, cloche, or even an old bedsheet on a cold night. In fall, the first frost often spares a warm microclimate for a week or two, so a light cover can steal extra ripening time from your tomatoes.
Your yard is not the airport
Official frost dates come from weather stations, which are often at airports on open ground. Your garden has its own microclimate. Low spots collect cold air and frost earlier; a bed against a south-facing wall stays warmer; a city yard runs warmer than the surrounding countryside. Over a couple of seasons you'll learn whether your garden runs ahead of or behind the published date, and you can adjust the SowWhen windows by a few days to match. Until then, the zone averages are a sound, conservative starting point.
Once the two dates are anchored in your head, the rest of the calendar stops feeling like memorization. Cool-season crops cluster around the frost dates; tender crops sit safely between them; and the length of the gap tells you whether a long-season crop like melons or winter squash even has time to finish where you live.
Build your planting calendar →