Sam, I have grown dahlias here in the Chicago area for many years. As you probably know, they are not necessarily winter hardy, depending on the severity of your winters, so this can be one factor. Normally here in Chicago we have to lift the dahlias and store them for the winter because the temperatures are just more than they can handle. As I grow them at the base of my greenhouse wall, I can leave them in the ground over the winter and they will regrow the next Spring, but not all varieties make it. Another complicating factor beside ground temperature and the depth that the ground freezes to is the amount of moisture in the ground. Dahlias will not tolerate too much water. If the ground retains more water in the winter/spring, they have a tendency to rot. Finally, as you suggested, some varieties are just less hardy than others. No one advertises this so you can only go on word of mouth experience. Bob Hoel 630-279-8510