Almost every fall planting decision runs backward from one number, and most gardeners misunderstand what that number is. Your first frost date is not a deadline, a forecast, or a promise. It is a probability from a thirty-year average, measured at a weather station that is probably not in your garden.
Understanding what it actually measures is what lets you use it properly: conservatively for crops you cannot afford to lose, aggressively for crops you can gamble on, and as a starting point you can move with a few dollars of fabric.
Where the number comes from
Frost dates on this site come from NOAA’s 1991 to 2020 US Climate Normals, which are computed per weather station from three decades of daily observations. For each station, NOAA publishes the dates at which the probability of a freeze having occurred reaches various thresholds. The commonly quoted first frost date is the 50 percent date: by that day in an average year, there is a coin-flip chance a freeze has already happened.
Three things follow from that, and all three matter.
Half of all years are earlier than your date. A 50 percent probability is not a safety margin. If you are timing something you cannot afford to lose, use a more conservative threshold, or take the published date and subtract two weeks.
The number is per station, not per ZIP. Frost dates are interpolated to your location from the nearest reporting station, which may be at a different elevation, nearer or further from water, and more or less urban than you are. A valley bottom three miles from a station on a ridge can be five degrees colder on a still night.
Thirty-year normals lag the present. The 1991 to 2020 window is the current official one. Most of the United States has shifted somewhat later for first frost over that period, which is the same trend that moved about half of US locations roughly half a zone warmer in the 2023 USDA map update.
Look yours up
Enter your US ZIP code for your USDA zone, frost dates & planting calendar.
Or browse frost dates by city for a major city near you.
First and last frost by USDA zone
These are the representative windows used to build the planting calendars across this site. Your own ZIP will vary, usually within a week or two.
| Zone | Min winter temp (°F) | Last spring frost | First fall frost | Frost-free days |
|---|---|---|---|---|
| 3a | -40 to -35 | May 15 | Sep 15 | 123 |
| 3b | -35 to -30 | May 13 | Sep 18 | 128 |
| 4a | -30 to -25 | May 10 | Sep 22 | 135 |
| 4b | -25 to -20 | May 5 | Sep 28 | 146 |
| 5a | -20 to -15 | Apr 30 | Oct 3 | 156 |
| 5b | -15 to -10 | Apr 25 | Oct 8 | 166 |
| 6a | -10 to -5 | Apr 20 | Oct 13 | 176 |
| 6b | -5 to 0 | Apr 15 | Oct 18 | 186 |
| 7a | 0 to 5 | Apr 10 | Oct 25 | 198 |
| 7b | 5 to 10 | Apr 3 | Nov 1 | 212 |
| 8a | 10 to 15 | Mar 25 | Nov 8 | 228 |
| 8b | 15 to 20 | Mar 15 | Nov 15 | 245 |
| 9a | 20 to 25 | Mar 1 | Nov 30 | 274 |
| 9b | 25 to 30 | Feb 15 | Dec 10 | 298 |
| 10a | 30 to 35 | Jan 31 | Dec 20 | 323 |
| 10b | 35 to 40 | Jan 15 | Dec 31 | 350 |
| 11a and warmer | 40 and up | none | none | 365 |
Notice that the frost-free column is not a smooth function of the zone number. Going from 6b to 7a adds twelve days; going from 8b to 9a adds twenty-nine. Warm zones gain season faster than cold zones lose it, which is why fall gardening changes character so sharply around zone 8.
Frost, freeze, and what each one kills
A light frost, 29 to 32 degrees, is the one the date predicts. It ends basil, beans, cucumbers, zucchini, tomatoes, peppers and eggplant. It does no harm at all to kale, spinach or carrots, and it improves them.
A hard freeze, 25 to 28 degrees, finishes half-hardy crops like broccoli, potatoes and cilantro, and knocks back the tops of root vegetables while leaving the roots usable underground.
Below 24 degrees, only the genuinely hardy crops carry on: kale, spinach, leeks, parsnips, garlic already in the ground, and mature carrots under mulch.
This is why frost tolerance, not days to maturity, decides how late you can sow. A crop that keeps producing for a few weeks past the first light frost has a later planting deadline than a faster crop that stops at the first one. The fall planting calendar by zone works that arithmetic out crop by crop.
Why your garden is not the weather station
Most autumn frost is radiational frost. On a clear, still night, the ground radiates heat to the sky faster than the air replaces it, and surfaces drop below the air temperature. Two consequences are useful.
Clear and calm is the dangerous combination. Cloud cover reflects heat back down, and wind mixes warmer air from above into the layer near the ground. A cloudy, breezy night at 33 degrees usually produces nothing. A clear, still night at 36 can frost a tomato bed.
Cold air is dense and flows downhill. It pools at the bottom of slopes, behind solid fences, and in hollows. The bottom of a slope can frost while the middle of the same slope stays clear, which is why orchards are traditionally planted on slopes rather than in the valley floor. Thermal mass works the same way in reverse: the south side of a masonry wall, a paved area, or a large water tank releases stored heat overnight and holds a few degrees.
Work out where your own cold spots are by walking out on the first frosty morning of the year and noting where the white is and where it is not. That map is worth more than any published date, and it does not change.
What actually buys you time
Floating row cover is the most cost-effective tool in fall gardening. Lightweight fabric gives 2 to 4 degrees of protection, medium 4 to 6, heavy 6 to 8. Two rules make it work: support it on hoops rather than draping it on the plants, because foliage pressed against frozen fabric still freezes, and seal the edges to the ground with soil or boards, because the protection comes from trapping ground heat.
Water is underrated. Soil that is moist stores and releases far more heat than dry soil, so watering thoroughly the afternoon before a predicted frost is worth a degree or two for free. Jugs or barrels of water in a cold frame do the same job.
Mulch protects roots rather than leaves. Straw over a carrot or parsnip bed keeps the soil unfrozen and lets you dig through December in a zone where the tops are long gone.
Cold frames and low tunnels are a step up, roughly one to two zones of effective shift for whatever is inside. An unheated greenhouse or high tunnel is the permanent version of the same idea, and it is the one legitimate way to garden outside your zone rather than merely at its edges.
A bedsheet in an emergency is genuinely fine for a single early frost. Cover in the late afternoon while ground heat is still present, not at 10pm, and take it off in the morning.
The last jobs before the first freeze
Harvest everything tender the day before a predicted frost: basil, beans, summer squash, cucumbers, peppers, eggplant. Pick mature green tomatoes rather than leaving them on the vine, since anything showing colour will ripen indoors and anything caught by frost will not.
Leave the crops that improve. Kale, carrots, parsnips, leeks and Brussels sprouts all get sweeter after a freeze, because they convert starch to sugar as an antifreeze response. Fall carrots left under straw are a genuinely different vegetable from summer ones.
Then plant garlic. The first frost is the start of its window, not the end of the season.