What does this fall planting calculator calculate?
This calculator works out the sowing window for each fall crop by counting backward from your median first freeze date. Cool season vegetables need to be in the ground weeks before frost arrives so they can reach harvest size while days are still warm enough for growth.
For every crop with a fall sowing window, the result shows the date range to sow, whether that window is open, upcoming, or already closed, and how many days remain in an open window.
What you will need
First freeze date
Enter the median first 32 degree freeze date for your area. If you have set your ZIP code, the calculator fills this in automatically using NOAA 1991 to 2020 climate normals from your nearest weather station, and it shows you the station name and distance.
The date is a probability, not a promise. Half of all years see the first freeze before the median date and half see it after, so treat every window as an estimate and watch the forecast.
How the calculation works
Each crop carries a fall sowing rule expressed in weeks before the first freeze. Lettuce, for example, can be direct sown 10 to 6 weeks before the median freeze. The calculator applies those offsets to your date to produce a sowing window, then compares the window against today.
A crop is marked "sow now" when today falls inside its window, "not yet" when the window has not opened, and "window closed" once the end date has passed.
Where the dates come from
The freeze date comes from NOAA climate normals for the station nearest your ZIP code. The crop timing rules are drawn from university extension planting guidance, including the University of Maryland Extension fall vegetable gardening recommendations.
Both are planning estimates. Elevation, urban heat islands, cold air drainage, and the weather in a given year can all shift the real date. A station miles away on a hilltop may freeze on a different night than your sheltered backyard.
How to use the sowing windows
Sow early in a window when your soil is still warm, since warm soil speeds germination and gives the crop more growing days. Sowing at the very end of a window works only in mild autumns or under row cover.
The windows describe direct sowing. If you start fall crops in trays and transplant them, you can move the outdoor date later because the seedlings get their first weeks under cover.
Worked examples
A mid-September check in zone 7
A gardener in Atlanta with a median first freeze of November 15 opens the calculator on September 20, about 8 weeks before frost. Lettuce, kale, spinach, beets, and radishes are all inside their sowing windows. Carrots, which need 14 to 10 weeks, have just closed.
A late start in zone 5
A gardener near Minneapolis with a median first freeze of October 5 checks on September 15, about 3 weeks out. Only radishes and spinach, with the shortest fall windows, are still open. Everything else has passed, so the realistic plan is a quick radish crop and row cover for overwintering spinach.
Counting backward for a specific crop
A gardener wants a fall carrot harvest. Carrots need roughly 14 to 10 weeks before the freeze. With a November 1 first freeze, the sowing window runs from about July 25 to August 22. Missing it means waiting for spring.
Common fall planting mistakes
Sowing on the freeze date, not before it
The first freeze is the finish line, not the starting gun. Crops sown too close to frost germinate but stall at seedling size.
Forgetting that growth slows in fall
Shorter days and cooler soil slow everything down. A carrot that matures in 70 days in June can need 90 or more in October, which is why the windows start so early.
Ignoring soil temperature
Many cool season crops germinate poorly in hot August soil. Shade the seedbed, water daily, or start seeds indoors and transplant if your fall sowing window lands in a heat wave.
When this calculator may not be the right choice
The windows assume a typical frost-free season. In mild climates where freezes are rare or absent, fall and winter gardening follows different rules and a local planting calendar will serve you better. In gardens where the first freeze has already passed, the results simply confirm that the season is over.
Frequently asked questions
Why do the fall sowing windows start so early? Fall crops must size up while days are still long and warm, because growth nearly stalls as daylight and soil temperature drop. A carrot that matures in 70 days in June can need 90 or more in October, so the window opens weeks before the freeze.
My crop's window has closed. Is it too late? For that crop, mostly yes. Sowing at the edge works only in mild autumns or under row cover. The realistic options are the fastest crops still open, like radishes and spinach, or overwintering crops under protection.
Can I move the dates later by transplanting? Yes. The windows describe direct sowing. Starting seedlings in trays gives the crop its first weeks under cover, which lets you set out transplants later than the last direct-sow date.
