First and Last Frost Dates, Explained

By The Boring Thing · Updated July 15, 2026 · 6 min read

Your average last spring frost is the date after which a killing frost is no longer likely, and your average first fall frost is the date after which one becomes likely again. The stretch between them is your growing season. Both dates come from NOAA climate normals—30-year temperature records—and both are probabilities, not guarantees. Knowing yours is the single most useful piece of information for deciding when to plant, when to harvest, and when to protect tender crops.

Frost versus a hard freeze

Not all cold nights are equal. A light frost occurs when the air temperature at plant height drops to 32°F or just below. It can damage or kill tender, warm-season plants—tomatoes, peppers, basil, squash—while leaving hardy greens and root crops untouched. A hard freeze, sometimes called a killing freeze, is generally around 28°F or colder for several hours. That level of cold damages a much wider range of plants and effectively ends the season for anything not specifically cold-hardy.

There is also a wrinkle worth knowing: frost can form on the ground and on leaves even when the official thermometer reads a few degrees above freezing, because air near the soil surface on a still, clear night is colder than the air at the standard measuring height of about five feet. Radiational cooling on calm, cloudless nights is exactly the condition that produces a surprise frost. So treat a forecast low of 34–36°F on a clear, windless night as a real risk, not a safe margin.

Where the dates come from: NOAA climate normals

Frost dates are not folklore. They are calculated from NOAA climate normals, which summarize 30 years of daily temperature observations at weather stations across the country. NOAA updates these normals each decade so they track a shifting baseline. From that record, statisticians work out the probability that a given location has seen its last freezing temperature of spring—or its first of fall—by any particular date.

This is why a frost date is always a probability rather than a line in the sand. When a source lists your "average last frost" as, say, April 30, it usually means there is about a 50% chance the last 32°F night has already passed by that date. Half of years, in other words, will still see a freeze after it. Some references also publish more conservative dates—a 10% chance of frost after that day, for the gardener who cannot afford to lose a planting.

What that means for how you plant

Because the date is a coin-flip midpoint, it invites a simple decision about risk tolerance:

• Risk-tolerant gardeners plant tender crops right around the average last-frost date, accept that some years they will need to cover plants or replant, and gain a longer season in exchange.
• Cautious gardeners wait one to two weeks past the date, giving up a little season length for a much lower chance of losing plants to a late freeze.

Neither is wrong. The right choice depends on how much a lost planting would cost you and whether you have covers ready. This is especially relevant for warm-season crops—our guide on when to plant tomatoes by region walks through exactly how to apply that cushion to a frost-tender favorite.

Growing season length

Your growing season is simply the count of frost-free days between the average last spring frost and the average first fall frost. A gardener in the upper Midwest might have 120 frost-free days; one on the Gulf Coast might have 280 or more. This number matters because seed packets list "days to maturity"—a rough estimate of how long a crop needs from planting to harvest. If a melon needs 90 days and you only have a 100-day season, you have almost no margin for a slow start, and you may want transplants rather than direct-sown seed. Knowing your season length turns "will this even have time to ripen?" from a guess into arithmetic.

Microclimates: why your yard may disagree with the map

Published frost dates come from the nearest weather station, which may be miles away and at a different elevation. Your own property has its own microclimate, and it can differ from the regional average by days or even a couple of weeks.

• Low spots and valleys collect cold air that sinks downhill on still nights—these "frost pockets" freeze first and thaw last.
• Slopes, especially those facing south, drain cold air away and warm earlier in spring.
• Cities and paved areas hold daytime heat and run warmer, often gaining a longer frost-free window than the surrounding countryside.
• Large bodies of water moderate temperature, buffering nearby gardens against sudden late frosts.
• Structures and fences can radiate stored warmth or block cold wind, creating warm and cold spots within a single yard.

Pay attention over a season or two and you will learn where frost lingers on your own ground. That local knowledge often beats any published date.

Pairing frost timing with soil temperature

Frost dates tell you when it is safe from freezing, but they do not tell you whether the ground has warmed enough for warm-season crops to actually grow. Soil warms more slowly than air, so it is common for the frost risk to pass while the soil is still too cold for tomatoes, peppers, or beans to thrive. Checking soil temperature is the companion step that keeps you from planting into cold ground that will only stall your seedlings.

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A quick reading at planting depth with an inexpensive soil thermometer tells you whether the soil has caught up with the calendar—take it in the morning, when soil is at its coldest, for the honest number.

How to find your frost dates by ZIP

Your frost dates depend entirely on location, so the fastest way to get accurate numbers is to look them up by ZIP code, which pulls from the nearest NOAA station data. Enter your ZIP into the free frost lookup tool above and you will get your average last spring and first fall frost, plus a planting calendar built around them. From there, adjust for your own microclimate and your personal risk tolerance.

One more note: frost dates describe when tender plants can be killed by cold, but they are not the same thing as your USDA hardiness zone, which describes how cold your winter gets and which perennials will survive it. The two are easy to confuse. If you want to understand the difference, see our guide to USDA hardiness zones explained—the short version is that frost dates govern planting timing, while zones govern which shrubs and trees make it through winter.

Putting it together

Find your average last and first frost dates by ZIP, count the days between them to learn your season length, then decide—based on your own risk tolerance and microclimate—whether to plant right on the date or hold back a week or two. Add a soil-temperature check for warm-season crops, keep a cover handy for surprise late frosts, and you will have taken most of the luck out of the equation.