Growing Dahlias In Pots Over Winter. You cannot keep your dahlias tubers in the ground over winter they will rot or die. Dahlias grow well in pots though you need to be very careful not to let them repeatedly dry out.
In hardiness zone 7 dahlias will usually survive the winter outdoors if the soil is well drained and you cover the area with a thick layer of mulch to insulate the tubers. Dahlias in only potting soil may grow poorly because potting soil is too light and porous to provide the rich moist soil these plants need. Dont forget to label the stumps so that you can keep track of each variety.
To avoid any risk of freezing its best to bring the tubers indoors.
Then put it in a paper bag in a cool dark place. Your dahlias will start growing again in spring. Oct 25 2016 329 PM CST. The process for growing dahlias in pots is a bit different from growing them in the ground.