I am answering this question generically as it has not had an answer. Dahlias like to be grown in raised beds and the soil that is used can be just about anything that is available. Raised beds drain better in wet weather and are slightly warmer than the soil level. Tall dahlias may be a bit more difficult to stake up as they are above you a bit as you work.

Thanks for answering. I thought this site had died! I am going to do raised bed with them. Can't fight the CO. soil. Thanks for caring.