I live in Prattville, zone 8. My dahlia bed is in full sun all day. I do make sure they get adequate water and fertilizer.
I have not grown them from seed, only plants or the tubers. My only problem is not taking the time to pinch them back and some get leggy where I have to put a plant frame around them. SOOOOO many colors and varieties. Just love them.
Lynn

I grew and showed dahlias for more than 10 years. It is better to have at least 1/2 day of sun and the more the better. In shade, they tend to grow tall and leggy with weaker stems. "Late" afternoon shade is OK to keep them from wilting so badly. I too grew them from tubers. The Dahlia Society of ALabama sells tubers at the Bham Bot. Gardens sale each year to help the BBG Auxiliary and to promote the plants. They will have prize winning cultivars for sale. Look it up on the BBG website.