A planting calendar is two numbers wearing a disguise: when the last spring frost usually happens, and how fast heat builds after it. Here is a 2027 calendar for Columbus, Ohio, built from NOAA’s 1991-2020 frost probabilities and degree-day normals for the Port Columbus airport station, with the same method for any town.
NOAA publishes three spring and three fall freeze (32°F) dates for each station, at 10%, 50% and 90% probability. For the Port Columbus airport station (1991-2020 normals):
| Probability | Last spring 32°F | First fall 32°F |
|---|---|---|
| 10% | May 5 (1-in-10 chance of a frost on or after it) | October 14 (1-in-10 chance of a frost on or before it) |
| 50% | April 18 (even odds) | October 27 (even odds) |
| 90% | April 4 (frost on or after it in 9 years of 10) | November 7 (frost by then in 9 years of 10) |
The “average last frost date” on seed packets is the 50% date, April 18 here, so planting on it means a frost after planting in about one year out of two. Waiting for the 10% date, May 5, cuts that to one year in ten. The frost date calculator pulls the same six dates for the station nearest any ZIP code.
The University of New Hampshire Extension’s planting chart (April 2016) times each crop from the average last frost and gives how long seedlings grow indoors first. Applied to Columbus for 2027, with the 50% date and, for a cautious gardener, the 10% date:
| Crop | Start seeds indoors | Plant out (50% date) | Plant out (10% date) |
|---|---|---|---|
| Broccoli | Feb 7 to Feb 21 | Mar 21 to Apr 4 | Apr 7 to Apr 21 |
| Lettuce | Feb 14 to Feb 21 | Mar 21 to Mar 28 | Apr 7 to Apr 14 |
| Beans (direct seed) | Sow outdoors | Apr 18 onward | May 5 onward |
| Tomatoes | Feb 28 to Mar 14 | Apr 25 to May 2 | May 12 to May 19 |
| Cucumbers | Mar 28 to Apr 4 | Apr 25 to May 2 | May 12 to May 19 |
| Peppers | Feb 21 to Mar 7 | May 2 | May 19 |
| Squash | Apr 4 to Apr 11 | May 2 | May 19 |
The cautious column moves every date by 17 days. That is the price of lowering the frost risk from even odds to 1 in 10: for tomatoes, setting out May 12 to May 19 instead of Apr 25 to May 2. Many gardeners split the difference by planting on the 50% schedule and keeping row covers ready until the 10% date.
Cool-season crops (broccoli, lettuce) go out before the average last frost because they tolerate light frost. Indoor start dates are counted back from the first plant-out date in each range. UNH also says to sow sweet corn when soil reaches 60°F, which no frost date can tell you; a soil thermometer can.
Frost dates say when it is safe to plant; growing degree days say how fast things happen afterward. Using NOAA’s monthly corn degree-day normals (base 50°F, 86°F cap) for the same station, spread evenly through each month, here is when corn planted on three dates would reach the 100 to 120 GDD that Purdue agronomists give for emergence:
| Planted | 100 GDD by | 120 GDD by | Days to emerge |
|---|---|---|---|
| Apr 18 (50% last frost) | Apr 30 | May 2 | 13 to 15 days |
| May 5 (10% last frost) | May 11 | May 13 | 7 to 9 days |
| May 20 | May 26 | May 28 | 7 to 9 days |
Planting in mid-April is not a head start in proportion to the calendar: corn sown on April 18 needs about 13 days to collect 100 GDD, while corn sown May 20 needs about 7, because late-May days are warmer. The growing degree days calculator adds up actual daily highs and lows, which beats the normals once the season starts.
Running totals from January 1 at the Columbus station (50/86 method, 1991-2020 normals):
| Date | GDD since January 1 |
|---|---|
| April 18 (50% last frost) | 268 |
| May 5 (10% last frost) | 429 |
| June 30 | 1,458 |
| July 31 | 2,215 |
| August 31 | 2,938 |
| October 14 (10% first frost) | 3,578 |
| October 27 (50% first frost) | 3,691 |
Between the 50% frost dates (April 18 to October 27) a normal year delivers about 3,414 GDD; between the cautious 10% dates (May 5 to October 14) about 3,141. Long-season crops and late plantings have to fit inside the smaller number in a cold year. For a fall crop, count its days to maturity back from October 14 rather than October 27.
The first frosts also bring the leaves down. They are the carbon half of a compost pile, and the compost calculator balances them against grass clippings or kitchen scraps to about 30:1 carbon to nitrogen and 60% moisture with Cornell’s composting equations.
| Place (NOAA station) | 50% last frost | 10% last frost | Tomatoes out (UNH) | 50% first frost |
|---|---|---|---|---|
| Denver, CO | Apr 30 | May 13 | May 7 to May 14 | Oct 12 |
| Atlanta, GA | Mar 24 | Apr 10 | Mar 31 to Apr 7 | Nov 8 |
| Helena, MT | May 9 | May 25 | May 16 to May 23 | Sep 30 |
| Dallas, TX | Mar 2 | Mar 19 | Mar 9 to Mar 16 | Nov 29 |
Helena’s tomato window opens in mid-May and Dallas’s in early March, a gap of more than two months, with the frost-free season roughly 144 days in Helena against 272 in Dallas.
At the Port Columbus airport station the 1991-2020 normals put the 50% last 32°F date on April 18; there is a 1-in-10 chance of a freeze on or after May 5.
Only frost-hardy crops. On the 50% date there is still an even chance of a later freeze; tender crops such as tomatoes go out 1 to 2 weeks later in the UNH chart, or after the 10% date for more safety.
They measure accumulated heat, so they predict how quickly seeds emerge and crops mature after planting, which calendar days cannot do in a cold or warm spring.
6 to 8 weeks before setting them out, according to the UNH chart; in Columbus that means late February to mid-March for an April 25 transplant.
A frost date is a probability, not a fixed day. NOAA gives the dates by which the last spring freeze has passed in 10%, 50% and 90% of years so gardeners can choose their own risk.