It’s that time of year, folks! Time to think about preparing and planting your fall garden. As all of you gardeners know, each planting zone has its own dates for fall planting. But do you know how those dates are determined? Better yet, would you like to be able to calculate exact planting dates for crops in your particular city?
In order to harvest crops in late fall or winter, they must be planted in late summer or early fall. I used to think the “drop-dead planting date” (my term for the absolute latest date you can plant) was determined based on the average first frost date. For fall and winter crops, minor frosts aren’t an issue. The planting dates are actually based on number of daylight hours.
In order to grow, plants need a minimum of 10 hours of sunlight every day. Once the daylight hours dip below ten, there is little to no plant growth. This period of time is called the “Persephone Period,” a term used by Eliot Coleman. (Eliot Coleman is a well-known American organic farmer, agricultural researcher and educator, and author of the books, The New Organic Grower, Four Season Harvest, and The Winter Harvest Handbook. His farm is in Maine, and he grows year round. I think he knows a thing or two. I plan to get all of those books and read them soon. They look tremendous!)
STEP 1: DETERMINE YOUR PERSEPHONE PERIOD
The key to calculating your last planting date is to determine the first day sunlight is less than 10 hours in your city. I found a great interactive calendar that will allow you to search by your city/state/country, and then will allow you to view monthly calendars with sunrise, sunset, and number of daylight hours. The calendar for Charleston, SC is here. (Just place your city/state in the search box.)
For Charleston, we have only two weeks where sunlight is BELOW ten hours a day – December 16-29. However, I worked my figures at the 10 hours 30 minute mark, just to be sure. Using that parameter, Charleston’s Persephone Period is November 12 – January 28. There would be very slow growth during that time period, with little to no growth the last two weeks of December.
STEP 2: DETERMINE YOUR LAST PLANTING DATE
The goal is to have your crops be 75% mature by the first day of your Persephone Period. (You can still harvest when there is little to no growth, but the level of maturity must be at about 75% at that point.) That sounds complicated, but it’s actually an easy calculation.
- All seed packages show the days to maturity of the plant. Multiply the number of days to maturity by 0.75 to get the number of days between planting and 75% maturity. For example, beets mature in 55 days. 55 days x 0.75 = 41 days (rounded to the nearest whole number). Therefore, it would take about 41 days for beets to reach 75% maturity.
- Next, subtract the 75% maturity days from the first day of your Persephone Period. In our example, beets would be 75% mature in 41 days. The first day of my Persephone Period is November 12 (using the 10 hr, 30 min mark). November 12 minus 41 days = October 3.
So, October 3 would be the last date I should plant beets. When I check my Zone 8 planting calendar for fall planting, it states that beets should be planted by October 1! Voila!
With this simple formula, you can now determine the last date you can safely plant any cool-weather crop for your city. All you need is the information on your seed packet and the daylight hours chart for your city on the website link above.