Dahlias are tropical plants, which means they will not survive the cold New York winter that is upon us. Since New York is in zone 7, they are treated as annuals or they are dug up and have their tubers stored in a frost-free environment (like your house) until the spring. Overwintering Dahlias is an easy task, and here is how to do it.
Wait until just after the first killing frost has passed, You will notice the top of the plants turning black. Cut the plants back to 4-inches and dig up a wide enough area to ensure including the entire root zone.
Remove the soil from root clumps by hand, but do not shake them; the plants are fragile.
Drying and Packing
Place the tubers in a dry basement, or outdoors on a screen rack or in a dry shady spot for a few days to dry. Gently wipe away any remaining soil from the tubers and tag them accordingly for different varieties, this way it will be easy identifying everything in the spring. Put four cups of vermiculite, sphagnum peat moss, wood shavings or sawdust into a gallon-size produce bag. These bags have pinholes spaced about a half-inch apart to allow for air circulation. Add tubers, then pack the bags into a wood, cardboard or plastic box, and cover it to ensure complete darkness. Store at 40-45 degrees. Check your tubers monthly. If you begin to notice any shriveling, sprinkle water onto the packaging material. If any of them are more than slightly shriveled, you can put them in a pail of water overnight, if they plump up– pat them dry and return to the soil. Discard any rotted or molding tubers, and air out the bag and box they were in. Expect to lose 10 percent of all tubers stored.
You can plant your Dahlias outside around Memorial Day. Place them in a spot that gets at least six hours of sunlight daily. To give them a head start, plant them in pots outdoors, kept by a sunny window, about a month before transplanting time.
Thank you Jessica Damiano for the original information. You can read more here.