Nothing seems to terrify a novice dahlia grower more than coping with the plants after the first frost. Coping with tubers does not need to be a trial. A professional grower can go to the following page for very detailed instructions on dealing with tubers: https://dahlia.org/docsinfo/articles/digging-dividing-and-storing-tubers.
A novice grower, or a casual gardener enjoying garden dahlias, however, has much easier options. One choice is to treat dahlias as annuals – throw away the plants after first frost and buy or rely on others for new plants each spring. A second choice, however, is to do the minimum to dig and store tubers:
- After frost, pick a day to dig, wash, cut, and mark clumps. Figure on being able to process around six plants an hour and limit yourself to a manageable number per day.
- Three to five days before the processing day, cut off the tops of the plants around 6 inches above the ground. Keep the label with the name of the cultivar on the stake. Throw the tops into the trash. (Home composting of dahlia trash is a bad idea – the dead foliage and stalks can have virus as well as insect and mite eggs that will survive composting.) During the few days between cutting off the tops and digging, the eyes, from which new sprouts will emerge next season, emerge enough that even a novice can identify them. Do not leave cut stalks too long, however, because rot can start in the crown and go down into the tubers.
- On the digging day, dig some clumps, keep the name with the clump, and wash it with a garden hose to wash away as much dirt as possible. Dig up to a dozen clumps at a time. Next cut the hair and feeder roots from the clump, and remove the remainder of the stalk above the crown. Leave enough crown so the eyes remain on the clump. Use a sharp knife to cut through the middle of the clump where the stalk was. Do this cutting twice to cut the clump into four parts. Wash well. Mark the four clump sections with the cultivar name. Let the clump sections dry for a day or two – either outdoors in the shade if the temperature will remain above 40 degrees F (above around 5 degrees C) or indoors in a cool room. Do not leave the clumps on concrete – a shoe box is a good choice.
- Clean your cutting tools with a solution of one part bleach to ten parts of water to minimize the likelihood of spreading disease from one plant to another. Clean tools after each plant.
- When the clumps are dry to the touch but not dried out completely, place them in a plastic grocery bag with vermiculite or wood shavings (hamster litter). There should be at least as much vermiculite or wood shavings as tubers in each bag, and ideally no more than one cultivar in each bag. Twist tie the bags and place the bags into a larger bag or cardboard box. Store in a cool environment that will remain around 40 to 50 degrees all winter.
- Check the bags once a month. Fungus usually means that the tubers were too wet when stored. If done properly, the quarter clump sections should not dry out. During most of the winter, the eyes are likely to disappear into the tubers. In late winter, the eyes will start emerging and making sprouts. When the sprouts emerge, one can frequently divide the quarter clumps into more tubers – but use the bleach solution when going from one plant to the next.
Hopefully these instructions are simple enough not to scare anyone away from cutting and keeping tubers. If you still find the instructions more than you are willing to handle, treat the dahlias as annuals and get new plants each season.