Gardeners living in temperate zones often feel that vegetables can only be planted during the frost-free periods. However, some of the cold hardy vegetables can be planted up to six weeks prior to the average last killing spring frost. Certain vegetables also can be planted late in the fall and in the winter months in these Temperate Zone climates to take advantage of their early spring germination and development. These cool season vegetables include lettuce, peas, spinach, broccoli, Brussels sprouts, carrots, radish & cauliflower.
Potatoes and other tubers and roots may freeze and rot during the winter. Hence, the planting of these crops is not recommended in this manner.
Tomato, squash, pumpkin and several other warm season crops frequently germinate early in the spring from fruit left in the garden from the previous fall. These ‘naturally’ seeded vegetables harden off quickly in the spring and are remarkably capable of withstanding freezing temperatures. They often out-produce expensive store-bought transplants set out after the last killing spring frost. Cool season crops seeded prior to or during the winter germinate in the spring based on soil and air temperature not the whim and fancy of the gardener.
Cool-season vegetables seeded too late in the spring are more susceptible to disease problems. A prime example is the powdery mildew that attacks peas during hot weather. Peas also yellow during these hot periods. Lettuce, spinach and Chinese cabbage that is not planted early enough to take advantage of the cool spring will often bolt (form a seed stalk) when hot weather arrives. Radish planted during the heat will typically bolt instead of developing an enlarged root. Planting in the fall just before the ground freezes allows these cold-hardy vegetables the opportunity to develop early in the spring when soil and air temperatures are ideal for this category of vegetable.