Dahlias are perennials that produce beautiful blooms throughout the summer. They continue blooming into late summer and autumn when other plants have stopped. Dahlia tubers can be treated as annuals and left in the ground over winter, or they can be dug up and stored over the winter in a cool place, and they will again produce blooms the following summer.
As soon as the first frost blackens the foliage, dig up the tubers from the ground and clean off. Cut off leaves, but leave a stem an inch or so long. You can wash off the tubers in water or wait until they are dry to remove the dirt. Some gardeners turn the tubers upside down to dry in the sun for a few hours to remove the moisture. It is important that the tubers be completely dry before storing them, as rot will destroy the tubers over winter. Leave tubers exposed to air for a couple of weeks. When the remaining stem is dried, it is OK to put the tubers into storage. You may wish to mark the tubers for variety and colour.
Use vermiculite, peat moss, dry shavings or coarse sand to store the dahlia tubers. You can get these at your local garden centre. You can place the tubers into paper bags or cardboard boxes for storage. Surround the tubers with the dry material. Make sure the tubers do not touch. Some gardeners prefer to dust the tubers with a fungicide before storing. The area where you store the tubers must hold a steady temperature between 4.44 to 10.0 degrees C. An old-fashioned root cellar is perfect for storage, as is a basement, crawl space, attached garage or even a refrigerator.
You should check your bulbs periodically over the winter to see that they are not too dry. If they appear shrivelled, you may slightly moisten the peat moss. Some growers soak the tubers in water if they are shrivelled, but make sure they are thoroughly dry before putting them back into storage.
Remove the dahlia tubers from storage in March or April. If you wish to divide the tubers, do so when the visible eyes are most prominent. These eyes will be future sprouts. Divide so that each piece has an eye. If no eyes are present, put the tubers in moist peat moss or other dry material and wait a couple of weeks. Eyes should appear. Pot the divisions in a soilless material, and plant outdoors when it is recommended for your area.