You should grow and pick spinach in the late spring or early fall when daytime temperatures are in the low 70s. It produces the best flavor when grown during cooler days. Mature spinach leaves can withstand frost.
Spinach grows from seed to edible product in six weeks. Spinach is a flower or bolt crop; if the temperatures are too hot, the crop bolts, or grows flowers and seeds instead of leaves. If you leave the spinach too long before harvesting it, the adult leaves become very bitter. Therefore, for best results, pick spinach every six weeks, no later than eight weeks after planting.