I live in Palencia, where sunflowers are now one of the principal crops. They are rotated between soybeans, rye, wheat, oats, and alfalfa, according to the soil type and market price. Depending on how much rain there is in the spring months, the big blooms start in late June and continue in waves, sometimes til September. They are left to stand and dry before they're cut, threshed, and sold -- mostly for sunflower oil, some for biodiesel.
The swallows almost always arrive for my birthday in early April, and the sunflowers bloom to mark my wedding anniversary on 13 July. (Yes, I am the center of the universe!)