Forget everything you learned about vegetables when you try growing spinach (Spinacia oleracea). The spring seeding usually recommended often gives disappointing results in Northern latitudes because the plant is genetically programmed to go to seed under the influence of lengthening days. By mid-May, the days are already long enough to stimulate flowering, ending its growing season. And as the days grow hotter in spring, the leaves become bitter and inedible.
It’s worth remembering that this vegetable originated in a Mediterranean climate, where summers are hot and dry and winters cold and rainy, and evolved as a winter-grower, sprouting in the fall with the return of cool temperatures and rain and reaching its maximum size in early spring before blooming in May. Modern varieties mature faster than the wild type, but still do better under short days, cool temperatures and a fairly moist soil. Spinach seeds are so immune to cold that they can even germinate at temperatures slightly below freezing!
In the South, spinach is best sown as a winter crop, that is, in late fall for harvest in the very early spring. In more northerly regions, too, the best results are obtained by sowing in the fall… but the way the plant grows will surprise you. The seeds will begin to sprout in the fall and the young plants begin to increase in size. Then, when the ground freezes, they stop growing, but remain in good shape during the winter, even under the snow, and then resume growth in the spring as soon as the snow melts. This gives you a very early harvest, earlier any other vegetable. Under the very harshest climates, it may be useful to cover spinach seedlings with a floating row cover. This added protection helps to moderate temperature extremes. Extra-hardy spinach varieties, like ‘Tyee’, are the best choices for overwintering spinach.