Spinach grows rapidly once it gets going, sometimes too rapidly. When it approaches peak maturity at times certain environmental conditions can cause it to quickly 'bolt' to seed before we are able to harvest. Increasing day lengths and Temperatures near or above 75 F will cause spinach to bolt.
If you are seed saving, bolting is fine, if not you need to harvest your Spinach ASAP or plow it under.
Bolt Resistant Varieties of Spinach
There are so called 'bolt resistant' varieties, but they too have been known to bolt to seed on occasion, just not as frequently, they are bred for more tolerance of unfavorable weather conditions.
Late spring and early summer planting.
Late spring to mid-summer
Low Bolt Varieties
Crop Rotation, Timing Spinach Planting for a continuous Harvest
Crop rotation of spinach is helpful in ensuring a continuous harvest of palatable spinach leaves. Cool season varieties in early spring and late summer and low bolt varieties in summer.
'For a continuous supply of spinach during the entire growing season, plant seed every week or two. Sow cool season spinach in the early and mid-spring. Then plant the bolt-resistant varieties Correnta and Spinner in the late spring and through most of the summer. Then follow with more plantings of cool season broadleaf spinach in the late summer and early fall.' Oregon State University
Hydroponic Spinach Strawberry Spinach Malabar Spinach
Spinach Compatible Plants How to Grow Spinach