Planting Your Dahlia Tubers or Cuttings
You can plant your dahlias as soon as the danger of frost has past, and the soil temperature remains above 55 degrees Fahrenheit. In Chicago, this typically means about the middle of May. However, some growers wait until Memorial Day just to be sure. The delay does not seem to appreciably affect when the first blooms appear. You can start with either potted plants or tubers that have been over-wintered from last year’s crop.
Read entire article: http://my.chicagobotanic.org/events/dahlia-primer/