At this time of year, it’s common for leafy greens to bloom or, as it’s known in garden-speak, bolt. Long hours of sunlight, combined with torrid temperatures induce flowering. In most cases, there’s nothing to be done. It’s simply time to pull the plants that haven’t yet been harvested and add them to the compost pile.
For example, spinach blooms when days last more than 14 to 16 hours. (Interestingly, spinach will only bloom when days are long.) Warm temperatures will accelerate this process. Dorothy Hinshaw Patent and Diane E. Bilderback explain why this happens in The Book of Garden Secrets: