Your last frost date is the average date of the last 32 degree morning at a nearby weather station, drawn from NOAA's 1991 to 2020 Climate Normals. It is a 30 year average, not a promise: about half those years saw a later freeze. Pair the date with the odds, the forecast, and your soil temperature before you plant.
That last sentence is the whole job, and most frost date articles skip it. They hand you a single calendar day for your zip code and let you believe the weather agreed to it. It did not. Below is how to read the number honestly, so a late freeze does not wipe out the plants you spent weeks raising.
These three words get used interchangeably, and that costs people crops. Each one does something different to a plant.
A freeze is a surface air temperature of 32 degrees or below. The National Weather Service issues a Freeze Warning when it forecasts a minimum of 32 degrees or less. That is the classic threshold behind your last frost date.
Frost can form when the official air temperature is a few degrees above freezing, roughly 33 to 36 degrees, on a calm, clear night. The plant surface radiates heat to the open sky and cools below the air temperature, so ice forms on leaves even when the thermometer reads 34. This is the range that triggers a Frost Advisory. A light frost like this may only tip burn the outer leaves of a hardy plant.
A hard freeze is the killer. It is roughly 28 degrees or below, held for a sustained stretch, and it ends the season for tender crops and most annuals outright. The exact number can vary a little by local NWS office, but the lesson holds: 28 degrees for a few hours is a different event than a brief 31 degree dip. Match your response to the severity. Do not panic replant frost tolerant crops over a single light frost, and do not leave tomatoes exposed to a hard freeze.
There is no national date, because the last freeze is entirely local. Much of the South is typically past its last freeze by the end of March. High elevations in the West may not clear 32 degrees until late May or even June. A rule of thumb pulled from a magazine is useless. You need the number for your specific area.
That number comes from NOAA's 1991 to 2020 U.S. Climate Normals, a database of 30 year averages maintained by the National Centers for Environmental Information. The last spring freeze date is simply the average date of the last 32 degree or lower reading at a nearby station. You can look it up through a NOAA station table, a NOAA based frost date lookup, or the planner on this site, which pulls the historical frost window for a location you enter.
One caveat worth knowing: the date is tied to a fixed station, often an airport, that may sit on higher, more open ground than your field. Hold that thought for the microclimate section, because it changes the number for a lot of growers.
Here is the part that actually saves plantings. NOAA does not publish one date per station. It publishes a spread of probabilities, and local NWS offices post these station tables. The three that matter are the 10 percent, 50 percent, and 90 percent dates.
The 50 percent date is the classic average last frost, the one you usually see quoted. After it, a freeze is historically less likely than not. In plain terms, planting on the average date is a coin flip. By definition, the last freeze in the record fell later than the average about half the time.
The 10 percent date is later on the calendar and far more conservative. Only about one year in ten saw a freeze after it. That is the date to use for anything frost sensitive and hard to replace, the plantings you cannot afford to lose. The practical rule is short: the average date is a coin flip, so if a freeze would wipe out the planting, plant to the 10 percent date or be ready to cover.
Frost dates have trended a little earlier in many regions as growing seasons have lengthened, but do not bet the crop on a trend. Any single spring can still throw a hard late freeze. Use the trend to plan and the forecast to commit.
Being past your last frost date clears the air, not the ground. The frost date governs air temperature. Soil temperature governs germination and root growth, and soil lags the air by roughly four to six weeks in spring. You can be two weeks past your frost date with soil still too cold to plant.
Warm season crops want soil at 60 degrees or warmer. Tomatoes transplant best into soil around 65 to 70 degrees. Below about 60 degrees their growth stalls and seed can rot in the ground. Cold, wet soil kills more early tomatoes than a late frost ever does. The plant sits there sulking, roots refusing to work, and a fungus finishes the job.
Checking soil temperature takes a cheap probe thermometer and two minutes. Push it about four inches deep, read it in the morning for a few days running, and do not sow warm season crops until it holds at 60 degrees or above. That single habit separates growers who lose flats of tomatoes every May from those who do not.
No. The last frost date is really two dates on one farm, one for hardy crops and a later one for tender crops.
Cool season, frost tolerant crops can go in weeks before the last frost date and shrug off a light frost. This group includes peas, spinach, lettuce, onions, the brassicas like broccoli and cabbage, and potatoes. They want cool soil and cool air, and waiting for the frost date wastes good growing weeks.
Warm season, tender crops should wait until after the last frost date and warm soil. This is tomatoes, peppers, eggplant, beans, squash, cucumbers, sweet corn, and basil. Rushing them into cold ground buys you nothing. A tomato set out two weeks early into 55 degree soil is often outgrown by one planted on time into 65 degree soil, because the early one stalled while the later one never stopped.
Because cold air behaves like water. It drains downhill and pools in low spots on clear, calm nights. Those low areas, the frost pockets, commonly run about 5 to 10 degrees colder than nearby higher ground, and they freeze earlier in fall and later in spring. If your official frost date comes from a station on open, elevated ground and your beds sit in a bottom, your real last frost is later than the number says.
Walk your ground on a frosty morning and you will see it. The hollows and the north side of a hedge hold frost while the slope above is clear. Know your low ground and plant it last, or plant it to hardier crops.
You can also buy yourself a margin. Floating row covers and cloches typically add about 2 to 5 degrees of frost protection on a cold night, often enough to carry a borderline planting through a marginal forecast. Cold air can even be encouraged to keep draining downslope by opening a gap in a fence or hedge at the low end of a plot, so it flows past rather than ponding on your plants.
The last spring frost is only one wall of the season. The first fall frost date, drawn from the same 1991 to 2020 Normals and the same 32 degree basis, sets the back wall. It tells you when the season closes.
This matters most for late plantings and second crops. Take a crop's days to maturity, subtract it from your average first fall frost date, and you know whether there is time to finish. If a 75 day squash needs to be in the ground by early July to ripen before frost and it is already late July, you have your answer. A quick first and last frost date lookup at both ends of the season keeps you from planting things that cannot finish.
Before you set out tender plants, run these four questions. If all four are yes, plant. If not, wait or protect.
Hardy crops clear this bar weeks earlier and can skip the soil temperature step. Tender crops need all four.
Every number above is local, so a generic lookup only gets you halfway. The planner on this site, the Farmer's Almanac Planner at /tools/farmers-almanac/, is built for exactly the comparison this article teaches. Enter your location and it returns your historical frost dates for the area alongside a 14 day frost risk flag based on the NWS minimum temperature forecast, plus moon phases, sunrise and sunset, growing degree days, and a 7 day forecast in one view.
That pairing is the point. It puts your average frost date and the actual next two weeks side by side, so you decide for this year rather than off a static number. Look up your area, read your historical frost window, then watch the 14 day frost risk flag before you commit tender plants. It will not predict the exact safe day, guarantee no frost, or read your soil, so keep the probe and the row cover handy. But for the average versus this year question at the heart of safe planting, it is the fastest honest answer you will get.
Your last frost date is the average date of the last 32 degree morning at a weather station near you, calculated from NOAA's 1991 to 2020 Climate Normals. It is local, ranging from late March in much of the South to late May or June in western mountains. Look up your specific area rather than using a national rule, and remember it is a 30 year average, not a guarantee.
A hard freeze, roughly 28 degrees or below held for a sustained period, is the killing frost that ends the season for tender crops and most annuals. A light frost around 32 degrees may only nip the outer leaves of hardy plants. Frost can also form on a calm, clear night when the air reads 33 to 36 degrees, because plant surfaces cool below the air temperature.
Not necessarily. The frost date clears the air, but tomatoes need warm soil, ideally around 65 to 70 degrees and no colder than 60. Soil lags air by roughly four to six weeks in spring, so it can still be too cold to plant even after your frost date passes. Check soil at four inches deep in the morning and wait until it holds at 60 degrees or above.
Planting on the average date is roughly a coin flip. By definition, the last freeze in the 30 year record fell later than the average about half the time. For crops you cannot afford to lose, plant to the later 10 percent date, when only about one year in ten saw a later freeze, or keep row covers ready to protect a planting through a marginal night.
Official frost dates come from a fixed station, often an airport on open, elevated ground. Cold air drains downhill and pools in low spots, so frost pockets on your land commonly run 5 to 10 degrees colder and freeze later in spring. Know your low ground, plant it last or to hardier crops, and use row covers, which add about 2 to 5 degrees of protection.
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