I am not sure in Alabama but I would guess that Jan/Feb depending on where in Alabama and what type of greenhouse you have.(heated or not) I started mine here in North Carolina in Feb.
The best way to check for any area is to go to that state's agriculture website. (Some states call is co-operative extension) Most states publish the best planting times. You can also give them a call.
Another way to check is to simply do a search on google. That's what I did first when I moved here from PA.