Question
as dahlias need to be lifted each fall, can they be grown in containers?  if so, how deep should the container be?  

Answer
Yes, Dahlias do very well in containers. As for the size of the container, my thought was, in proportion to the size of the plant. But that is a very general answer. Dahlias come in many different sizes. The larger ones will need a good size container, not only for their roots but for a weight balance when the top gets tall and heavy. I would say the best way to judge is to put the tuber in the middle of the container, and see if you have an inch on all sides. Do this every year when you pot the plant up to a larger container.
I prefer plastic containers, because they are lighter, therefore easier to handle. They hold the moisture well. And they are less expensive.
Use a good potting soil, with very good drainage. Don't water the tubers all winter. Store them in a dry, dark place. If they seem too dry, moisten them lightly. In the spring, bring them out into the light, give them light watering, until you see good growth started. Then let them dry between heavy watering. Don't fertilize until the plant has flower buds. Look for a fertilizer for flower bloom, and use a light application on the first of every month, all summer, with the last one on August first.
When the plant begins to go dormant again, decrease the water gradually. The tuber should last many years, if you take good care of it.

bakerplanter