菜花 is ambiguous — in mainland China it usually refers to cauliflower (花椰菜), a white or green vegetable, while in other contexts it can mean rape flower (油菜花), the bright yellow blossoms of the rapeseed plant. In Taiwan it's also a slang term for genital warts, so be careful with context. When you mean 'cauliflower' in everyday speech, 花椰菜 or 菜花 are both common; for the flower fields, say 油菜花 instead to avoid confusion.