Dahlias are tuberous plants and naturally become dormant for the winter as the weather gets colder and the days shorten. It is possible to extend their season a little by bringing them into a heated greenhouse and putting them under horticultural lights but they will still require a dormant period. To get the best from them over-winter the tubers by lifting them, brushing off any soil and placing them somewhere frost-free to dry out. Once they are dry store them in containers or paper bags but keep checking them throughout winter for signs of rotting. In February pot up the tubers and start growing them in your a heated greenhouse under lights again. This should certainly give them a head start. As soon as the risk of frost has passed in late spring or early summer plant them outside.