I planted broccoli plants (aprox. 5") in 2nd week of June and the plants have grown to almost 1ft. At the same time, the center stalk has little yellow flowers sprouting out of it. Does this mean that it has gone to seed and it is too late to harvest any stalks. The stalks don't seen to be mature at this point (only the size of a pencil). Should I cut the center stalk out and let it keep going? Thank you for your input.