Dig up and store your dahlia tubers over winter
After a frost has blackened the foliage on your dahlias is a good time to prepare them for winter storage. Depending on your area, this could be in late September or some time in October.
Cut the stalks down to about 6"-8". Wait a week and then carefully dig up the tubers. Waiting a while before digging ensures that the tubers have formed eyes for next year.
If you are expecting a hard freeze, then dig them immediately after cutting the stems. I like to use a potato fork to dig them up, though a shovel will do if you are careful where you dig. Take the fork or shovel and stick it into the soil about a foot from the center of the stem and rock it back and forth to loosen the soil. Do this all around the stem. Once you have loosened the soil, you can easily lift the tubers by the stem, using the fork as leverage to push the tubers up out of the ground.
Once the tubers are dug, shake off any excess soil and rinse them well. Allow them to dry thoroughly on a screen, turning them occasionally, in an enclosed, frost free area that gets good ventilation. Make sure there is no water inside the stems. At this point, you can cut the stem shorter, if you like. After about a week, I cut the tubers apart, keeping the ones that have not been damaged and that have visible eyes, then return them to the screen to finish drying. You can also cut them apart at the time of the initial digging, if you prefer. Drying time can be shortened if the humidity is very low, so check them every day before they start to shrivel.
Once dry, the tubers can be stored in dry peat moss, sand, vermiculite, sawdust, or wood shavings in a cool dry place such as a cellar or garage. Gallon vegetable storage bags from your local market work great because there are already little tiny ventilation holes in the bag. You can always make your own holes using an awl, if necessary. Sprinkling them with a fungicide or powdered sulfur will ensure a better rate of survival. Temperatures should be around 40-50 degrees F for storing. If stored at a higher temperature, shriveling and/or rotting can occur. If stored at lower temps, then the tubers could freeze and turn to mush. Make sure you mark the bag with the name of variety you are storing and put them into a box with a lid so that no light can get to them. Any type of container, made from any type of material will do as long as it does not let light into the container. I've had good success with storing them in a cooler. The cooler also helps to keep the temperature constant when the exterior temps may fluctuate from time to time.
Inspect the tubers every few weeks and remove any diseased or mushy tubers. If you notice any that have started to shrivel, you can either sprinkle a little water on them, spray them with water using a spray bottle or take them out, soak the tubers in water over night, dry them as you did before and put them back into the peat moss. Do not expect a 100% survival rate, but 80% or better is very likely under optimum conditions.