My reaction is to say "NO". The plants need enough time to establish their roots before the frosts want to kill them. You will need strong plants to survive the winter. Depending on where you are in eastern WA, you might be able to get away with it with a good mulch layer and/or a hoop house, or just grow them indoors, and wait for the last frost next spring to x-plant them outdoors.
My plants are pretty frost hardy, but a freeze down in the mid-20's or lower will knock them back to the ground.... So I would go with John's advice and grow them up indoors or in a tub over the winter for planting out early next spring.
I started from a 4" pot of a single cutting in the Spring of 2011, divided the plant in Spring of this year, and divided a couple more of the younglings that were encroaching on a path a couple months ago. I'm up to ~50 plants now at last count.
If you just chop the crown and roots up in a tub of potting (or even garden) soil, they will start to put out more roots and top growth within a couple weeks. If this was done in a cool/cold garage, they may slowly develop root&crown over the winter.
"Limitation is the mother of good management", Michael Evanari
Location: Southwestern Oregon (Jackson County), Zone 7
Are they tiny lil root cuttings? If so; a 4in pot will do for over-wintering... If they are large root cuttings, then I'd suggest more like 8~9in pots...
I agree that the garage might be a good place to get them started; or perhaps on the floor somewhere in the house... Once the foliage breaks the soil surface, they will need some light. Given that your winter sun is even less than ours, I would place in a south-facing window that received "sun all day" (well, you know what I mean)...
After the last frost; dig a hole that is a minimum of 6in wide by 12in deep. Cut the foliage down to one inch stubs and place in hole. Back-fill with a (cool*) manure and a bit of good compost. Place plant in hole 1~2in below soil surface. Add 1~3in of mulch on top of that (leaf mold mixed with small woodchips works super)
*Goat, deer, llama, alpaca, rabbit or any other cool to semi-cool manure such as equine
Well, I'm much furthur south, so I hope I do well. Just 3 days ago my co-worker came in with a root cutting for me. I went nuts. She claims its illegal to have comfrey in our state, but I don't know that, except that she said it. Highly doubt it the comfrey police will show up anyway. I planted mine outside, in a pot, but I will take it in if its gonna get too cold. Hope I do well. I'm taking a coarse in Herbal Studies right now and I think I can fix a lotta stuff with comfrey.
Roses are red, violets are blue. Some poems rhyme and some don't. And some poems are a tiny ad.