From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-1905-671243
Question: I planted several broccoli plants a month ago. They were
growing beautifully, but now the heads of broccoli are covered with
yellow flowers. Why, and what do I do?
Answer: Sounds as though your plants bolted. Broccoli grows best in
cool weather. When it gets too hot the flower buds that actually make
up a head of broccoli quickly open up into the yellow flowers you see.
There's no way to save your plants once they've bolted. Try planting
again in late summer for a late winter harvest.