Sowing Planting Calculation

HOW TO CALCULATE FALL GARDEN SOWING/PLANTING DATES
Objective: Bring cool-weather crops to maturity (semi-hardy crops) or, at least, near-maturity (cold-hardy crops) before the sooner of a) the first average frost/freeze date or b) November 18 (which is the last day in Sonoma County with 10 or more hours of daylight).

STEP 1

Days-to-Maturity (DTM) for crop variety:		______________
Add 7-14 days for shorter/cooler days:	+	______________
Fall DTM:		_________days*
STEP 2
First freeze date (32°; 50% probability):		______________
Critical daylight date for Sonoma County:		___November 18
Earlier date of the two:		______________
STEP 3
Step 2 date:		______________
Subtract Step 1 Fall DTM	-	______________
Last sowing/planting date:		______________

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IF APPLICABLE:

Is this crop normally transplanted or direct-seeded*?

a) If a normally transplanted crop is, instead, direct-seeded,
ADD germination days to the DTM + ______________

b) If a normally direct-seeded crop is, instead, transplanted,
SUBTRACT germination days from the DTM - ______________

*See “D” and “T” (direct-seed and transplant) notations for crops on the MGSC Publication Year-Round Food Gardening in Sonoma County.

BACKGROUND INFORMATION:

The DTM (days-to-maturity or harvest date) can be found on the seed packet. If purchasing a seedling to transplant and the plant tag does not have this information, google the crop variety. The DTM is based on when the seed or transplant “hits the soil” in your garden.
The DTM on a seed packet or online is based on spring planting conditions when the days are becoming longer and warmer. However, when planting in the fall, days are becoming shorter and cooler. Therefore, we add 7-14 days to our DTM.
Select varieties with appropriate days-to-maturity (i.e., varieties that, from the date you sow seed or plant in your garden, will come to maturity or near-maturity before the first frost/freeze occurs OR daylength is less than 10 hours—whichever is the earlier date).
You can view NOAA’s weather “normals” online to find first and last average frost date probabilities for the weather station closest to you. If you live in Sonoma County, the Master Gardener Food Gardening Specialists have extracted the important dates. See First and Last Frost Dates. Be aware that NOAA frost dates are based on 30-year averages and are updated every ten years (the next update should happen in 2021 or later). Use the NOAA data along with conditions in your own microclimate and current year weather predictions. [Note: if you use garden websites for the “first average frost date,” they will cite NOAA’s 50 percent probability for 32 degrees – i.e., 50 percent chance that there will be freezing weather before that date. Take this into consideration based on your own conditions and the frost-sensitivity of crops selected. If you are very risk-adverse, you might use the 10% probability date . In Santa Rosa city that is November 15 vs. December—which would impact your calculations.]
Cool-weather crops stop growing (or grow unperceptively) with less than 10 hours of daylight. This date depends on your latitude. In Sonoma County, this date is November 18. Ten or more hours of daylight resumes on January 23 in Sonoma County (this period is also known as the “Persephone Period”—a reference to Greek mythology). If you live outside of the county, you can find daylight information on a Sunrise and Sunset Calculator (type in your city/state/county; then scroll down for daylight statistics).
IF YOU DECIDE TO SEED A CROP THAT IS NORMALLY TRANSPLANTED OR YOU DECIDE TO TRANSPLANT A CROP THAT IS NORMALLY SEEDED, there is one more calculation you need to make.
- First, see “D” and “T” (direct-seed and transplant) notations for crops in the MGSC Publication Year-Round Food Gardening in Sonoma County. These notations indicate the usual seed company designations (note that a few seed companies will occasionally stray from this standard).
- If you direct sow a crop that is “normally” transplanted, add the germination days to the DTM.
- If you plant a seedling of a vegetable that is “normally” direct sown, subtract the germination days from the DTM.

June 2021