Answer: It sounds like your spinach is beginning to "bolt" or set seed, so you might as well go ahead and harvest all of it now. (Once it sets seed it doesn't taste very good any more. In addition, warm weather can make it taste bitter.)
Next time, you can begin harvesting the outer leaves as soon as they are large enough to cut. The plants will continue growing as long as they have adequate nutrients and moisture and you will be able to enjoy the harvest over a longer period of time. Spinach is basically a cool weather crop, best planted and enjoyed either in early spring or planted in late summer for a fall harvest, so you may want to wait until late summer before you replant.
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