First Frost Date Calculator
Find your average first fall frost by zone, and the last planting dates it imposes.
The first fall frost is the deadline the whole late season runs against. Every July sowing of beans, every pumpkin on the vine, is a bet placed against this date. The calculator gives you the average first frost for your zone, the growing time left from any date you choose, and the last sensible sowing date for a crop you pick.
Like its spring twin, this is a zone average: half of all years frost earlier than it. Plan the crops you care about with the buffer built into the dates below, and find your zone on the map with the Planting Zone Finder if you need it.
How this is worked out
Each zone carries an average first fall frost from the same climate basis as the hardiness map, with half zones shifted about 5 days (the a half frosts earlier in fall, the b half later). The last sowing date for a crop = first frost, minus the crop’s days to harvest, minus 14 days. The buffer does double duty: it covers the half of years that frost early, and the slowdown of fall growth under shortening days, which extension services often price as a fall factor of one to two extra weeks.
Assumptions:
- Averages hide the spread: plan tender crops to the cautious deadline, hardy crops to the frost itself and beyond.
- Days to harvest are packet figures grown in good conditions in long days; the buffer absorbs typical fall slowness, not a neglected bed.
- Frost at plant height in a hollow arrives before frost at the official thermometer. Low corners of the garden live in a colder zone than the map says.
Worked examples
1. Zone 6b, bush beans. Average first frost Oct 27 (zone 6 average Oct 22, plus 5 days for the b half). Beans at 55 days plus 14 of buffer: last sensible sowing about Aug 19. A Sep 1 sowing is a gamble on a late frost year.
2. Zone 5a spinach. First frost about Oct 3. Spinach laughs at light frost, so its real deadline is set by day length, not cold: sowings into early September still size up before growth stalls in November.
3. Zone 8b tomatoes already growing. First frost near Nov 25. The question flips from sowing to ripening: fruit set by late September has the 60 to 70 warm days it needs, and green fruit at first frost ripens indoors off the vine.
What survives the first frost, and what does not
| Group | At first frost | Notes |
|---|---|---|
| Basil, beans, squash, cucumbers, tomatoes (plant) | Killed or near it | Harvest ahead of the forecast, not after it |
| Peppers, eggplant | Damaged, then killed by a hard freeze | Pick everything usable at the warning |
| Carrots, beets, turnips | Unharmed; sweeter after frost | Mulch heavily and harvest through winter in mild zones |
| Kale, spinach, chard, leeks | Unharmed by light frost, hardy for weeks | Cover extends them deep into winter |
| Winter squash and pumpkins | Fruit damaged by frost | Harvest when the rind is hard, before the frost date |
A frost (32 F) and a hard freeze (28 F or below) are different events about two weeks apart in most falls. Tender foliage dies at the first; roots and hardy tops wait for the second.
Common mistakes
- Sowing to the average with no buffer. Half of all years frost before the average. The two week buffer is the price of finishing the crop in a typical year.
- Ignoring the fall slowdown. A packet’s 55 days is counted in June light. The same variety sown in August needs about 65. Gardeners call the difference the fall factor.
- Leaving tender harvests out ‘one more night’. The forecast frost that gardeners gamble on is the one that arrives. Pick the tomatoes; green ones ripen on the counter.
- Dumping the whole garden at first frost. Half the garden is just getting good. Roots sweeten, kale sweetens, and covered greens run for weeks after the tender crops are compost.
Putting the result to work
Count back from this date for every late sowing with the Succession Planting Planner, which stops each crop’s repeats at the last date that can still finish. And when the frost actually threatens, the dig list for tender bulbs lives in the Tender Bulb Overwintering Scheduler.
When is my first fall frost, and what does it cut off?
- Pick your USDA zone (the Planting Zone Finder has it if you do not).
- Optionally pick the crop you want to fit in before the season ends.
- Read the average first frost, the cautious deadline two weeks earlier, and the crop’s last sensible sowing date.
- Count late sowings back from the last sowing date, not forward from today.
- When frost is forecast, harvest tender crops ahead of it and cover the hardy ones you want to keep.
How accurate is a zone first frost date?
As with the spring date, it is a planning average: the real first frost lands within about two weeks of it in most years, earlier in half of all years. Plan tender crops to finish two weeks ahead of the average and the odds are firmly on your side.
What is the difference between a frost and a freeze?
A frost is 32 F at plant level, enough to kill tender foliage like basil and beans. A hard freeze, 28 F or below for some hours, kills half-hardy plants outright and ends most of the garden. They typically arrive about two weeks apart in fall, frost first.
Why do fall crops take longer than the packet says?
Days to harvest are measured in the long, warm days of late spring. In fall, shorter days and cooler soil slow growth, so gardeners add a fall factor of one to two weeks to packet figures for anything sown after midsummer. The last sowing dates here include a two week buffer for exactly this.
Which vegetables actually improve with frost?
Many roots and brassicas: carrots, parsnips, kale, Brussels sprouts and leeks convert starch to sugar as a antifreeze response and taste sweeter after light frosts. They are the reason the garden is not over at first frost; with mulch and a cover, they harvest for weeks or months after it.
Can I extend the season past the first frost?
Yes: fleece and low tunnels buy about two to four weeks at each end by holding a few degrees on clear cold nights, and cold frames do more. Covers protect against frost, not against the dark: by late fall, day length, not temperature, is what stops growth in northern zones.
Zone frost averages are planning figures consistent with the 2023 USDA Plant Hardiness Zone Map climate basis; half zones shift the whole-zone average by about 5 days. The two week planning buffer covers early-frost years and the standard fall growth slowdown.
