In northern (or southern) latitudes, vegetable growth virtually ceases when day-length becomes less than ten hours. Days are shorter than ten hours for at least a few days any place on the globe above latitude 30 or below latitude -30. This includes almost all the United States except the most southern parts of Texas, Louisiana and Florida. The calendar months during which there are fewer than ten hours between sunrise and sunset have been dubbed the “Persephone period” by Eliot Coleman. Although plants grow little during the Persephone period, mature plants may remain healthy. Thus, winter harvest of many salad greens is possible if they are planted in Fall, allowing enough time for them to reach maturity before the beginning of Persephone.
Johnny’s Select Seeds has published a chart that helps chose, depending on the date at which Persephone begins at your latitude, the best Fall planting date for various salad greens to allow winter harvest. However, the chart is awkward to use. Based on this chart, I have created a free database that automatically calculates the best Fall planting dates for your location. To use it, sign up for a free Airtable account here. Once you have signed up, download a copy of the Fall Planting for Winter Harvest to your Airtable account here.