If you have divided your dahlia tubers, lay them down on the moistened layer of peat moss and make sure they are not touching (if one succumbs to fungus, there's no point in having it spread to the others).
Cover the dahlia tubers with another layer of peat moss.
Store the box in a dark spot that will remain cool for the winter but will not freeze. For many, this means an unheated basement or cellar.
Finding a place to store dahlias at a suitable temperature is one of the most problematic aspects of the whole storage operation. The optimal storage area will consistently retain a temperature above freezing but below 50 degrees throughout the winter. Some people simply lack access to such a storage area. Rather than going through all the trouble of digging dahlias, etc. as described in this tutorial, such folks should treat them as annuals, buying new tubers each year.
Even if you think you have done everything correctly, Mother Nature can intervene and rob you of success. For example, one year, I had a frost in late October, and I dug my dahlias shortly after. So far, so good. I took all the steps described in this tutorial and put my dahlia tubers in their winter-storage space. Problem was, winter missed the memo and did not arrive on time. November, usually a cold month where I live, ended up being downright summery at times. The result? The storage space I had selected for my dahlia tubers was too warm, and I lost them.
Other Tricks Used for Dahlia Storage
Some people wrap their dahlia tubers in newspaper before storing them in the box filled with peat moss. Still others store them in paper bags.
Here is a way to facilitate the disinfecting process (by dusting) discussed on Page 5. Insert some peat moss in a Ziploc bag. Place the cured dahlia tubers in the same bag. Follow this up with a liberal sprinkling of sulfur dust. Seal the bag. Grasp it in both hands. Gently shake the bag around (upside-down, then right-side up, then upside-down again, etc.), in order to distribute the sulfur dust around. By "gently," I mean you want to move the sulfur dust around, not the tubers. The peat moss will cushion the ride for the tubers some, helping you to avoid injuring them. Remove the tubers gently from the Ziploc bag. They are now ready to be stored in the box filled with peat moss.
How do you choose between all these variations on storing dahlia tubers? Well, maybe you should not be too hasty about narrowing your choices down to one method. Until you hit upon a method that works for you, why not experiment with a number of methods each year? That way, you may be increasing your chances of preserving at least some dahlia tubers over the course of the winter.
Experimentation affords another benefit: you're learning. We gardeners understand the concept of "continuing education," because nature is always throwing some new puzzle at us that we must figure out, even if it's only how to control some new weed we've just identified.
Label the dahlia tubers so that you will know what specific cultivars you are working with when next spring rolls around. Use a system of marking that is best suited to your own situation. If you have lots of clumps to store away, consider using separate boxes (else you should fasten a labeling tag firmly to each clump, to make sure it is properly marked).
Periodically check the dahlia tubers during the winter. If they seem overly dry, spritz the peat moss again with the spray bottle. If, on the other hand, a dahlia tuber feels mushy when you check it, it has rotted and should be discarded.
Storing dahlias away in peat moss is, alas, a project laden with melancholy. It's one of those unmistakable signposts that mark the end of the growing season. But on Page 10, we turn to a happier thought: what to do with the dahlia tubers in spring....