Hi Leanne,

Please accept my sincere apologies for this delayed response. To answer your questions -

The key to successfully storing dahlia tubers for the winter is ensuring that they stay dry, have good air circulation, and are in a cool, dark spot; they cannot be too wet, too dry, or freeze. The ideal dormancy storage temperature for dahlias is between 40 and 50 degrees Fahrenheit, but that can be tricky to find within a heated home. Freezing temperatures ruin tubers, and higher temperatures encourage microorganisms and fungus and prompt the tubers to develop sprouts too soon. The best places close to this temperature range are a root cellar, basement, garage or attic.

There are a number of materials to store dahlias over the winter including vermiculite, wood shavings or sand. They all can work, and it usually comes down to personal preference. The material should help conserve moisture within the tubers while at the same time preventing fungus and rot. The tubers in selected material can be placed in Styrofoam, wood, or cardboard containers. If one uses a wood or cardboard box, moisture could escape so in order to retain the moisture, use newspaper at least eight sheets thick to line all sides of the container and keep the top of the container closed. Remember to occasionally check (every 3 weeks) on the dahlias over the winter and remove any tubers that have started to rot before the decay spreads to healthy tubers.   If they seem too dry, mist them lightly.

If you don’t have a suitable area to store the tubers in a container, the following link suggests wrapping in plastic wrap and then store in a refrigerator. It is a “new approach” and most experts recommend against this method since the tubers will be more susceptible to rot. If it is your only choice, you may attempt it but it may not be successful:

       https://monroe.cce.cornell.edu/resources/fall-dahlia-tips

Regarding rinsing the tubers, it is recommended to rinse the tubers since it removes any clumps of soil that might be holding a lot of moisture and microorganisms which could potentially lead to rotting in storage. Additionally, rinsing makes it easier to divide the tubers.  Once they are rinsed, the tubers need a period of curing, or drying - place the tubers out of direct sunlight in a dry, well-ventilated area with the temperature is 60-70 Farhenheit for 1 to 3 days.
 
As far as dividing in the spring or fall, you really could do it either way. Most commercial dahlia growers divide in the fall when the tubers are more tender and easier to cut. It also makes them easier to store and quicker to plant in the spring. The only issue is that it is harder to find the critical eye which is found at the top of the tuber where it joins the stem. Many people cut the big clumps into manageable smaller clumps of whole tubers and those can be divided in spring when the eyes are easier to see (and many have started to sprout). Bottom line, there is absolutely nothing wrong with storing whole clumps and making your divisions in the spring. Either way will work and it comes down to personal preference. The American Dahlia Society has excellent instructions on dividing tubers

https://www.dahlia.org/docsinfo/articles/digging-dividing-and-storing-tubers/ and photos can be found on https://www.dahlia.org/wp-content/uploads/2020/09/Processing-tubers-pictorial.pdf

An additional website with useful information:

https://hort.extension.wisc.edu/articles/storing-tender-bulbs-for-winter/

I hope this helps and happy gardening!