Fall Planting Formula - Clarification from a Hort Expert

A few weeks ago, I had a post entitled Formula for Success – planting Fall vegetables, in which I tried to decipher a formula I found online that should help in deciding what crops could still be planted in the fall.

The formula that I found was this:

+ Number of days from seed to transplant if you grow your own

+ Average harvest period

+ Fall Factor (about two weeks)

+ Frost Tender Factor (if applicable); 2 weeks

= Days to count back from first frost date

Alan McDaniel,  Associate Professor Emeritus, Department of Horticulture,  Virginia Tech, was kind enough to provide this more detailed explanation: 

Who would think that a math degree is needed for gardening, but it is sometimes true. 

Let's look at the components of your equation, with a small modification. If we plant in the "normal" spring growing season, we can plant seed of a tender crop at the spring frost date. That takes into account a safety factor for a slightly later freeze when a tender plant is germinating and still protected under the soil surface. (Of course, a tender-crop transplant needs planning for some type of freeze protection since it does not have the protective soil cover.) We estimate the first harvest from that crop with the number printed on the seed label. For example, a "65 day" crop needs about 65 days from seeding to first harvest, after which we have a short or long productive time, or harvest period, depending on the crop, management, etc. All of this is based on warming, longer days of spring into summer. 

Shifting this to a fall harvest time, we need to think instead about how the crop responds as the days are getting shorter and cooler. This is the fall factor. Thus, a 65-day crop from seeding could need closer to 80 days (although germination will likely be faster since the soil is warm at seeding). That is the fall factor in your equation getting us to the first expected harvest time. How long that harvest continues depends on the crop involved. Something like bush beans we could look at for a single, once-over harvest, but your preference could as well be for multiple harvests over a two-week period. Thus, the average harvest period will vary with every crop (and gardener). 

When we bring the frost factor into the picture, it helps us understand when cool-season crops (half-hardy and hardy) are better choices for the fall garden because a) they generally thrive under the cooler temperatures (with shorter days) and b) the frost factor is less a worry.

Thanks so much for the detailed explanation, Dr. McDaniel. I enjoyed our conversation.