Spinach is a cold-weather crop. When it is planted in the spring, it tends to bolt, or go to seed, even when the crop is relatively young. By the time warm spring weather arrives and the spinach is growing nicely, the days have become long enough to initiate flowering. This is why you see the flowering stage come on so quickly. You need to plant earlier in the spring or even try planting at the end of summer for a fall harvest.