It looks like your spinach is "bolting" or flowering. Bolting is caused by long daylight periods from late spring to early fall. The recommendation from UC Davis, Vegetable Research and Information Center is to choose varieties carefully and to plant in fall or early spring. http://anrcatalog.ucanr.edu/pdf/7212.pdf says, "if the crop is allowed to become overmature, the stem elongates and produces a seedstalk with narrow, pointed leaves. This seedstalk formation (bolting) is caused by late-season high temperatures and long days."