If you live in a mild climate such as we have here in the Pacific Northwest, most years you can get away with leaving your dahlias in the ground if your soil is well drained and you mulch deeply to protect from frost. However, there are three good reasons for not leaving your dahlia tubers in the ground. In a really hard winter some dahlias will be lost to a hard freeze. If your soil is not well drained, you may lose some to rot. If you leave your dahlia tubers in the ground the clumps continue to increase in size, sending out more shoots each spring thus weakening the stems and decreasing flower size. Digging and dividing allows you to protect your tubers from the elements, increase your stock, rejuvenate the plant and share tubers with friends and relatives.
Dig your dahlia clumps a week or two after the first hard frost of fall. The plants will blacken and become unsightly so cut off all the stalks to about 6" above the ground. Leave your clumps in the ground for at least a week before digging. This will force the eyes to develop and will make dividing much easier. If your area does not have a hard frost by late October or early November, then cut back your plants and start digging the following week.
Dig your dahlias with a garden fork or a shovel. Shovels work better in heavy, wet soils; garden forks are better for dry, sandy soils. Shovels will also shear off longer tubers rather than spearing them as the fork might do. Sheared off tubers are fine. Many people cut off the ends of long tubers anyway to make them easier to store. Dig down on all four sides about a foot away and lift clumps carefully so as not to break the necks of any long or thin-necked tubers. Wash the excess dirt off the clumps with a garden hose and trim off all feeder roots and rat tails (long tapered roots at the tuber ends that look like a rats tail). Tag or label your clumps if you wish to keep track of the variety. Allow clumps to dry overnight in a cool dry location. Avoid leaving in the sun.
Use by-pass hand pruners, garden scissors, lopping shears, and a sharp knife to divide your clumps. Prevent knife cuts by wearing a protective fillet glove on your off hand. These can be purchased at cutlery stores and are used by the fish and meat industry to protect the hands from severe cuts when using very sharp knives.
The first step in dividing your clump is to remove all easy to separate tubers. These are obvious to see as they may be higher on the stalk or protrude noticeably from the crown (swollen area at the base of the stalk where the tubers form and where the eyes can be seen). Remove these tubers with a sharp knife or with garden scissors. Next, using lopping shears, cut off the stalk of the clump as close to the crown as possible. Then using the lopping shears again or your hand pruners split the clump in half. This will give you two easy to work with pieces of tuber clump. You should now be able to easily divide the halved clump by using your hand pruners, garden scissors or knife. When using the knife, get a short piece of 2x4 to rest the halved clump on while cutting. Make sure you leave an eye with each division. Eyes are only located near the crown of the clump and can usually be located with a little practice. You should get several tubers from most clumps, although very skinny tubers often don't make it through the winter. The final step in dividing your dahlia clump is to clean up the ends of each removed tuber. With your knife and scissors, cut off as much stalk tissue as you possible can without harming the eye of the tuber. The stalk tissue of a dahlia is different than the tissue of the tubers and is more susceptible to rot. Remove as much of this material as possible. Your tubers are now ready for storage. Also, to prevent the spread of virus, clean your tools with water and dishwashing detergent before going on to divide your next clump.
Soak the divided tubers in a bleach solution (one cup bleach to three gallons of water) for 15 or 20 minutes. This solution will kill any fungus or bacteria that might be lurking on any of the tubers and will prevent its spread during storage. An easy way to do this is to place all of the tubers you get from one clump or from one variety into a two-gallon plastic nursery pot and then place the pot into a five gallon plastic bucket containing your bleach solution. After 15 or 20 minutes, remove the pot, let the water drain back into the larger bucket for reuse. Let the tubers dry for a few minutes and while they are still slightly damp write the name of the variety on each tuber using a Noblot Ink Pencil #705 ("A bottle of ink in a pencil"). These pencils can be purchased at most art or office supply stores. You can also label your tubers with a permanent marker pen after the tubers are thoroughly dry.
After your dahlia tubers have completely dried for a day or two, place 5 to 8 tubers in a 1-gallon zip lock storage bag along with 2 or 3 cups of coarse vermiculite.* Quart size bags work well for 4 or fewer tubers or very small tubers. What vermiculite does better than any other storage medium (sawdust, sand, potting soil mix) is to absorb excess moisture given off by the stored tubers. Seal and store bags of tubers in cardboard boxes or plastic bins in a dry cool place until spring. Use layers of newspapers for added insulation if necessary. It's very important that your storage area does not freeze. The ideal temperature for stored dahlias is between 40 and 45 degrees. Check on your stored tubers once a month to make sure they are not rotting or shriveling. If they seem too moist then repack with dry vermiculite. If they are drying out then spray a little water into the bag. Any rotting material should be removed immediately. The ideal storage location would be an old root cellar. Most of us, however, will have to make due with a less efficient location. The crawl space under the house, an unheated but insulated garage or even an old unplugged refrigerator will do.
*Some vermiculite has been found to contain particles of asbestos, so be advised to wear a dust mask when handling this material. Alternatives to the vermiculite would be sawdust, damp sand or cheap potting soil right out of the bag without moisture added. All of these alternatives have drawbacks; however, they may be worth a try. In any case, do not use peat moss, as it will dry out the tubers.