Hopefully you've had a successful and rewarding dahlia season! It's always sad to see the frost nip them when they just seem to be going strong. Once the frost hits, you have to decide if you want to go through the effort to save them. If so, it's time to dig the tubers and store them for winter.
Here's my techniques for digging and storing dahlias.
Cutting & Digging:
I wait until after the first hard frost to cut mine down using a big lopper, leaving about 3-4 inches of stem to grab hold of.
Then, using a pitch fork, I loosen the soil around the tuber and gently pull up from the stalk, releasing it from the soil. If you notice your tubers are all tangled and deformed, it means that your soil is very compact. It would be helpful to add more compost next year to have looser soil for them to grow.
To Wash or Not:
If your soil is very compact around your tubers, you can wash the soil off now and let the tubers dry out a bit in a garage. Make sure to do this in the shade or they will start to dry up and shrivel. Once they are dry, you can move on to storage.
If you have sandier soil or very dry soil on your tubers, you can brush off as much dirt as you can and put the tubers into storage that way. It doesn't matter either way. The preference is just in the ease of dividing the tubers in the spring. I had stored mine with dirt on it but find it harder and more laborious to get the dirt off in the spring. I'm switching to washing mine off in fall.
Dahlia growers all seem to have their own way of storing- finding what works best for them and their situation so take my advice and adjust to what works best for you.
Before you start with storage, make sure to label your tubers and your bins if you grow more than one variety. I use one bin per tuber variety and write the tuber name onto recycled plastic knives or spoons and tuck them in the crates.
Once your tubers are ready for storage, you will need to keep them in a location that does not get below 45 degrees (50 is ideal) like a root cellar or unfinished basement. We keep ours in a shed that is semi insulated and keep a remote thermometer in the room to keep track of dipping temps. Tubers will rot if they freeze or if they are kept too humid.
You can store them in crates or boxes or even paper bags. I have used plastic bins, but find that they don't breathe and can rot due to the moisture buildup.
To help prevent moisture buildup, try packing the tubers in pine shavings, wood shavings or peat moss. I have done this for years and find it helps wick moisture in the bins.
Some growers do not use any medium and store them just in bulb crates. Keep an eye on them if you do store them this way, so they do not start to shrivel as not enough moisture can cause them to dry out.
As I mentioned, find a way that works best for you and test it.
Once spring comes, you will be ready with a whole new stock of tubers to start over again!
Check out my Blog on Dividing and Propagating Dahlias.