Two things limit what market gardens can grow in the winter; cold temperatures, and lack of sunlight. Cold temperatures are not nearly the big problem they might seem to be, as many vegetables, particularly including many types of salad greens, tolerate freezing temperatures quite well if given just a little protection from the weather. But there is no getting around the fact that plants require sunlight to grow, so even salad green growth slows to a snail’s pace when day length becomes too short in the winter. A rule of thumb is that growth nearly stops once day length becomes less than ten hours. The winter period when day length is less than ten hours has been dubbed the Persephone period by winter gardening guru Eliot Coleman .
Thus, a common farming strategy is to plant vegetables for winter harvest in the Fall, on dates that are carefully chosen so that plants will become almost fully grown precisely at the start of the Persephone period. Such plants, if given proper care, will remain healthy and ready for harvest far into the winter.
Johnny’s Select Seeds publishes a remarkably useful calculator chart,Planting Dates for a Winter Harvest | Johnny’s Winter Growing Guide (johnnyseeds.com). But it is tedious to use the chart as it requires manually calculating each planting date based on the date of the beginning of the Persephone period, which differs according to latitude.
With this in mind, I have created a spreadsheet version of Johnny’s chart, which uses Excel’s arcane DATE functions to do the required calculations automatically, once your Persephone date has been entered at the top of the spreadsheet.