Answer: I've done the same thing with my dahlias - left them in the pots in an unheated garage over the winter months. But, the tubers do multiply so you can only do this every other year or the roots will be overcrowded and the plants won't grow and bloom well. What I would do is leave them in place until the first frost kills back the tops of the dahlias. This will signal the tubers that it is time to rest. Cut off the tops right at ground level and set the pots in the garage. The potting soil should be fine for two or three year's growth. When the time comes to dig and divide the tubers you can dump the potting soil into a wheelbarrow or other large container and mix in some new potting soil. I mix about 50/50 new and old potting soil. When you divide the tubers you can plant two in a single large container - two different colors, if you like. Aside from the above suggestions, it sounds as though you are doing everything right. Enjoy your dahlias.
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