"Spinach is a cool season crop that readily goes to seed in warm weather and as days lengthen. It's best grown as an early spring or fall crop. It's still OK to use the leaves on plants that have gone to seed, but they usually don't remain productive for very long once they start seeding.
Variety selection plays a role, too. In my own garden, Oriental Giant is going to see right now while right next to it, Tyee and Melody are not."