When dividing tubers I do not recommend using a knife--although it is tempting, because it is too easy to have the knife slip and you will end up with a severe cut or injury. I use “lobster scissors” and garden clippers at needed for each clump. Occasionally I will have to use a knife but ALWAYS cut away from your hand or body. Remove all "rat-tail" sized roots and all roots from each tuber. Discard any damaged tubers and any that do not contain a visible eye.
I use a fungicide called Daconil to kill fungus on the tubers. After dividing the tubers place them in a bucket with ¼ cup of Daconil in 2 gallons water and soak for a minimum of 20 minutes.
Next, remove the tubers from the bucket (wearing rubber gloves) and put them on ‘raised’ trays to keep the tubers out of water. Leave tubers overnight in the trays with a heater and fan running to dry them. The next day place the tubers in micro-perforated bags, then put into 28- quart plastic containers for storage and kept in the cool dark potting shed for the winter.
Some growers recommend that you place the tubers in a bed of sawdust or vermiculite, inside a cardboard or wooden box. I, once again, do not use this method because you cannot see a tuber that may be rotting that will cause the rest of your tubers to rot as well. I store all tubers in clear micro-perfed bags so that once a month I can easily check to see if there are bad tubers and remove them if need be.
Store the tubers in a dark dry area where the temperature will remain at about 40-45 degrees F. If you park your car in your garage and intend to keep your tubers in the garage, they will shrivel and dry out from the heat that comes from your car’s engine. A true root cellar is too moist. Your basement is too warm. An outside woodshed or pump house would be best for tuber storage--especially if it's insulated.
Check your tubers periodically during the winter for signs of rot. I wipe away the moisture that accumulates on the lid of the boxes. I’ve found that putting a couple of dry paper towels will absorb moisture and keep the dahlias from getting wet and rotting. Water and moisture is the dahlias worst enemy.
An excellent video on dividing dahlia tubers is also available on YouTube:
I'd be happy to answer any questions you may have concerning 'my' method. Thanks! Jan
HOW I DIVIDE AND STORE DAHLIA TUBERS: