You do not want broccoli to flower. That means it is going to seed and will not form heads. Broccoli grows best in temperatures between 75 - 85 degrees. If the temperature goes much warmer, you need to try to keep the ground beneath your plants as cool as possible to keep it from going to seed, or flowering. You can do this by mulching around the plants and giving them extra water. You want heads to form, rather than flowers. When the head is 4 - 7 inches across and the individual florets are about the size of a match head, the broccoli is ready to be harvested.