Spinach germinates and grows best with temperatures in the 40 to 75 degrees Fahrenheit range. Though spinach tolerates light frosts, chilly air slows the growth rate of leaves and inhibits flowering. As long as temperatures remain cool, the soil evenly moist and day lengths less than about 14 hours, spinach plants produce lots of leaves for harvest. About eight weeks after germination, spinach may produce flowers if environmental factors are conducive, reports "Sunset Western Garden Book."
Bolting of spinach may be temporarily delayed if leaves are continually harvested by cutting plants back to about 1 inch tall. Plants rejuvenate with fresh leaves. Failure to promptly harvest, as well as long days and temperatures above 80 degrees Fahrenheit, causes plants to quickly produce flower stems and bloom. The warm temperatures also cause the flavor of spinach leaves to become bitter and the texture tough and leathery. Therefore, once a plant produces flowers, stop harvesting leaves, pull up the plant and discard it.
When to Grow
Depending on climate, gardeners sow spinach in fall, winter or spring to take advantage of the naturally shorter days and chilly temperatures. In the Northern United States, sow spinach seeds in mid-August for a fall crop up until frost or in late March or April for a late spring harvest. In southern states, sow spinach in mid-September for mid to late fall crops and in February for a mid-spring harvest. In subtropical areas, sow spinach in fall or winter.
Choosing Spinach Cultivars
Garden centers and mail order catalogs sell various cultivars of spinach seed. Look at the days to maturation information on the seed packet or product description. From 35 to 80 days are needed for spinach plants to grow enough leaves for harvest. If your cool season is short, choose cultivars that are harvest-ready in 40 days or less. Also look for cultivars that are described as "slow-to-bolt" or "long standing." This means the spinach tolerates warmer temperatures and doesn't produce flowers as quickly in the warming days of spring.