Just last night I was reading the 1962 The Prairie Garden and there is quite the article about Dahlia culture written by Bjorn Peturson.

Harvesting Dahlias

In southern Manitoba dahlias are usually cut down by frost during the second half of September. In some mild seasons, with a little protection, dahlias may survive into October.

Dahlias should be dug about 10 days after the first killing frost. The 10 days between the death of the top and lifting of the roots allows the roots to mature somewhat and to develop a covering more impervious to water loss.

If liberal amounts of peat and barnyard manure have been added to the soil the plants will lift quite easily without appreciable root injury. After the clumps of roots have been cleaned of soil and dried for about 3-5 hours they are ready for winter storage.

Storing Dahlias
The dahlias can be stored as undivided clumps and the clumps then divided into individual roots in the spring or the clumps can be divided in the fall and the roots then stored much as one would store carrots.

If they are to be stored as clumps some of the soil should be removed from the clumps and these placed in boxes which have been lined with water proof paper and packed in sawdust, sand, dry soil or peat soil. The boxes should be stored at a temperature of 40-50 degrees (we didnÂt have Celsius back then) in a well-ventilated room.

The writer has had good results with the following storage method.

1. The soil is cleaned from the clumps immediately after digging and the clumps are then divided into individual roots, making sure that each roots has a bud. The small side roots and the tail end are cut off each root with a sharp knife and all cut surfaces covered with dusting sulphur.
2. The roots divisions are dried for a few hours and then placed in a ventilated plastic bag. Each bag is only about half filled and the unused portion rolled around the roots. The roots are then placed in wooden boxes and covered with sawdust. The roots are examined periodically during the winter.

Sharon

I also let the tops freeze while the plants are still in the garden. Wait a week, then I dig out the whole clump and put them and their labels into individual containers. I like using the stackable "holy" black plastic crates that I can get at the nurseries for $1 or $2 that their plant orders come in, in the spring. I let the clumps dry, soil and all and remove the dead foliage down to an inch or two above the soil. I leave these crates, stacked in the garage until frost is likely in the garage and and then bring them inside. This also gives the soil around the root balls a good chance to dry out. Then I place and stack the open containers into my storage area in the basement that I keep the potatoes, carrots, canna lilies, calla lilies, etc. in for the winter. I find that the dahlias will be sprouting by Feb/Mar and by keeping the original soil on the roots they do not dry out and wither away, like they do if I divide the dahlias in the fall. I clean up the root ball in the spring and divide it then.

I use the same storage procedure with my cannas and callas and begonias. Some of these kinds of bulbs are in plastic pots for placing on my deck. In the fall I let them get a killing frost, cut the frozen foliage off, make sure the planter is dried out and throw the whole planter into the storage area in the basement. When I bring out the planter in Feb/Mar, the bulbs in the planters are just thinking about sprouting and all I do is water them and add a little slow release fertilizer into the soil. Every two or three years I will dump out the cannas out of the pots in order to divide them...also done in the spring.