Though leafy greens might seem too fragile for cooler temperatures, fall is in fact the best time to plant them in most United States hardiness zones. Many varieties prefer cooler weather.
Overall, the biggest considerations in planting leafy greens are the timing and type. Cooler areas such as zones 5 and 6 should focus on cold-loving greens. These include spinach, claytonia, mizuna, arugula and winter lettuce. Germinating the seeds indoors during the summer for planting in August and September is helpful. Warmer zones, such as zone 7 and higher, can start their seeds in late summer for planting in October or November.
However, if you live in a warmer zone like the Southwest, Gulf Coast and South (hardiness zones 7, 8, 9 and 10) and want to give spring planting a try, plant heat-tolerant greens like greens Malabar or New Zealand spinach in late spring. Just keep them shaded during the hottest months with shade cloth.
Thanks to stable and relatively temperate conditions, many varieties of leafy greens can be planted almost year-round along the West Coast.
However, in general gardeners throughout the contiguous United States will want to avoid planting leafy greens in the summer. Temperatures are usually too high for the seeds to germinate effectively.