Do you usually dig up your tubers or not? This late in the season, I wouldn't worry about anything underground. Just cover any holes in the stalk/stump remaining and either dig up when it's cooler or leave in the ground and protect as usual. I've had dahlias do this every once in a while and the tubers do fine. It may try to send out some new shoots if our warm weather holds, but I'd either ignore them til they freeze or break them off as they sprout.

Did you have your Bashful staked? Sorry it broke for you, but hopefully you got to enjoy it for a few weeks.

I usually leave them in the ground here in Southwest Washington state. Then in the spring I dig them up and divide. But I had not seen such a woody trunk on any other dahlias so this is why I ask. I am new too growing the collarette types. We have been enjoying these blooms for nearly two months.

You might want to consider taking cuttings from the main stem--there have to be cuttings because you said it was a wide plant. I would remove all but3 or 4 of the topmost leaves--do not leave any big leaves. If big leaves are the only leaves available, cut (with scissors) at least half of the leave off--you want to leave only enough for the plant to survive, and hopefully grow roots. I would take at least 3 cuttings and do as above, and try rooting them in water.
I would also take 3 or 4 more cuttings and try to root them in a "soil". Do NOT use your regular garden soil--it is wonderful in the garden, but is not for potted plants. I would make a mixture of 4 parts sifted garden soil, 4 parts of a peat/perlite mix like sunshine brand, 1 part each of Vermiculite, perlite,and sand(I use pool sand, brown because it is cheaper than the white--I suggest you see if your source has any broken bags, and if so offen them half their regular price. Be sure to take a container with you.)

OR
You can make a soilless mix of equal parts of perlite, vermiculite, milled sphagnum moss, and 1/2 part sand. If you have it use 1 part of the contents of a Britta Filter.(It is charcoal, and an excellent addition to the above.)
You may think I am trying to overload you, but if you would sterilize the first suggestion--before the OR,-- after you have it mixed by placing the mixed contents in a container you can place in a microwave, dampen it, and set it on high for up to 5 minutes or until your meat or candy thermometer reaches 185 degrees (NEVER PLACE THE THERMOMETER IN THE MICROWAVE!!!!!)you will increase you chances of success 100% ! Yes, I know, it takes time, but you sound as though you really like this dahlia.
Place the cooled mixture in the container you are going to use to root the cuttings. It would be best if you would "Tent" the container with some kind of plastic to keep the humidity high, but don't let the leaves touch the plastic or you will lose them, and create a potential mold situation. I will gladly answer any questions you may have. Write me at the "address" above. Also, Kindly put the word "GardenWebber" in the subject area to keep your e-mail to me out of my junk pile. My filters are set high because of spam. If I get your message I will respond.
GOOD LUCK !!!!!!!!!!!!!!!!!

Corgicorner here again. I forgot to suggest that you apply a rooting medium to the stem ends before inserting into either the "soil" or "soilless" planting mediums.