I am not sure why that is. It is a plant that likes warmer weather. I believe the zones for growing it as a perennial are 7 and higher. It should grow as an annual in your zone and it could be something with the seeds. Do you or have you tried growing them in sun directly next to your house? I know some grow them that way in colder zones. Many say the white is the most vigorous in colder climates but I don't know if that's the case.
