Frost Date Math

Fall Planting Calculator: Find a Latest Sowing Date

By Tam Vu · Updated

On this page
  1. How the date is calculated
  2. Example
  3. Station or manual date
  4. Sources

Plan a fall crop by counting backward from the first fall freeze. Enter the days from transplanting to harvest from your seed packet or plan. If you raise transplants indoors, add the days they need before transplanting; if your packet's day count already starts at sowing, leave that extra field at zero to avoid counting the seedling period twice. The calculator uses the first-freeze date from a nearby NOAA station or a date you enter.

Find a latest sowing date

Enter timing from your seed packet. The calculator does not supply days to harvest or a harvest window.

How the date is calculated

University of Nebraska–Lincoln Extension's fall-gardening method counts back from the first fall frost. This calculator applies its components:

latest sowing date = first-frost date − days to harvest − seedling days − harvest window − 14-day Fall Factor − 14 days if the crop is frost-tender

The default is the 50% first-freeze normal, the average date used in UNL's fall-planting method. You can choose 10% for an earlier, more cautious date or 90% for a later date. The 14-day Fall Factor is UNL's allowance for fall growing conditions. The additional 14-day Frost Tender Factor applies only to the tender crops identified in the guide. The days to harvest, seedling period and harvest window come from your own seed packet or plan; the calculator leaves those choices to you. UNL's frost-tender examples are corn, beans, cucumbers, tomatoes and squash.

Example

Suppose a garden uses a 50% first-freeze normal of October 27, and a seed packet says 60 days to harvest. With no indoor seedling period, no harvest window entered and the crop not marked frost-tender, the calculation is 60 + 14 = 74 days backward. The resulting example date is August 14. If the crop is frost-tender, UNL's additional 14-day factor moves the example to July 31.

These dates illustrate the arithmetic, not a recommendation for a particular vegetable. The packet's day count and your chosen station/date drive the result. For the meaning of the station probabilities, see first frost dates. For UNL's crop groups and the different NC Extension classifications, see frost-tolerant vegetables.

Station or manual date

Choose a state and the NOAA station closest to the garden. The calculator starts with the 50% average normal; 10% is earlier and more cautious, while 90% is later. If the calculated sowing date has already passed, the result says so and remains available as a planning reference. A climate normal is not a forecast, so check the local forecast as the season approaches. You can also enter a date yourself if you already have one from a local source.

Sources

  • University of Nebraska–Lincoln Extension, Fall Vegetable Gardening (G1343), for the Fall Factor and Frost Tender Factor.
  • NOAA National Centers for Environmental Information, U.S. Climate Normals 1991–2020, annual/seasonal station data.