This has happened to me a couple of times in the South too, with various cole crops. Have you grown this variety of broccoli in this location and at the same time of year before, with success? The reason I ask is, some brassicas require vernalization (a sufficient number of cold temperatures before they flower/head) and some types may be photoperiod sensitive (waiting for days to get short enough/long enough to trigger flowering/heading). Did you have an especially warm winter? Definitely make sure you're growing a variety that's best suited to your latitude, and are growing it at the suggested time of year. I hope you at least got to eat your plants! The leaves are a delicious consolation prize. Also! Important tip: always leave broccoli roots in the ground when you harvest the plants. Decaying broccoli roots are toxic to verticillium dahliae, a stubborn soilborne fungus. Broccoli is worth growing for that reason alone!

- colleen