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Because dahlias are tropical plants, they won't survive winter outdoors in New York. So, here in zone 7, we either treat them as annuals or dig up and store their tubers in a frost-free place until spring. If you grew dahlias you'd like to enjoy again next year, overwintering them is easy. Here's what to do:
Wait until just after the first killing frost (you'll notice the top of the plant turning black) and cut plants back to 4 inches and dig up a wide enough area to ensure including the entire root zone.
Remove soil from root clumps by hand, but don't shake them; they're fragile.
Drying and packing
Place 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 any remaining soil from tubers and tag them, if you have different varieties, for easy identification 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.
Pack the bags into a wood, cardboard or plastic box, and cover it to ensure complete darkness. Store at 40-45 degrees.
Check tubers monthly. If you notice any beginning to shrivel, sprinkle water onto packing material. If any of them are more than slightly shriveled, you can try to resuscitate them by soaking them in a pail of water overnight. If they plump up, pat dry and return to storage. Discard rotted or molding tubers and air out the bag and box they were in. Expect to lose 10 percent of all tubers stored.
Plant your dahlias outside around Memorial Day, in a spot that gets at least six hours of sunlight daily. To give them a head start, plant them in pots indoors, kept by a sunny window, about a month before transplanting time.