Is September too late to divide recently-started comfrey?

I'm in S.E. Michigan. I received 2 comfrey plants that were in sad shape. I planted one which now has 8 new leaves. The other, I divided in half and planted. The halves now have 4 and six leaves. These plants were transplanted about 4 weeks ago. They didn't have a lot of root, only about 3 inches if tap root and some tangled regular roots (not sure what "regular" root are called). My question is, is it too late in the year to dig one up and start a bunch of new plants off of the tap root. I saw a video that said you can start a plant off the smallest bit of root. So I would like to cut a root into 10 pieces and plant them along my back property line away from the garden where they are now planted. Will get any growth by dividing them now? How much root do I need to use? Can they be in morning and half afternoon sun and then shade from around 3pm on? Or should I put them in pots till next spring?

If you have large enough roots that you can take 10 large pieces from a plant, you could probably do it now despite the season.  Comfrey is an extremely hardy plant that seems to be able to survive almost anything; people have a hard time killing it. You might find more success with your project by waiting until the spring when the roots are wanting to grow like crazy.  Right now, they are probably heading towards dormancy, and are putting out the opposite energy.  A black barked  (as opposed to a new lighter colored less barky one) comfrey root that is between the size of a pencil and a small finger will likely reproduce clone style readily. A three inch piece of larger than pencil size has worked for me.  Anything smaller than that might, but might not, as they have to have enough juice for the new growth.  If your plants are small, and just divided, you might want to wait a year extra and really get good quality larger cuttings; you will be able to take more, and still have abundant plants.  

My comfrey bed gets a lot of shade.  

My house shades them in the morning till about 11 a.m.  They get an hour or two of direct sun then the sun goes behind an oak and there is filtered sun till 3 or 4 p.m., then an hour of direct sun, then more filtered shade.  There is some direct sun very late in the p.m.  Thus is all in late spring through late summer.

Now that the sun is lower in the sky, the poor plants get more filtered shade than sun.  When the trees lose their leaves there'll be more sun but by then the comfrey will be dormant.

I live in northern California.  The comfrey do well here with mild winters and hot, very dry summers.  Possibly the shade is an advantage to them.

After typing all that, I just remembered the two plants I have on the north side of my house.  They get no direct sun except for a couple hours in very late spring through early summer, when the sun is highest in the sky. I think your plants will do OK.

Dar,

It's so late in the season that dividing the plants won't do much more than give you relatively weakened plants going into the winter.  I'd plant out what you have, give them a good mulching with shredded pine mulch, and a good drink and leave them alone until spring.  Once they break dormancy in the spring, you can start to aggressively propagate.  You can take root pieces as small as 1" and the diameter of a pencil and produce new plants. You can take the chance of sticking them in the ground but you greatly increase your propagation success ratio by putting the cuttings in 4" pots with a very loose potting mix (if the water doesn't drain quickly, the cuttings will rot). Keep them well water but not so much that the potting mix resembles that of pots sitting in a bowl of water. The porosity of the mix should stop any tendency to water-logged mix. Keep in indirect sunlight, ie, no direct sun.  When you see roots at the drain holes you can plant out.  Often leaf growth is fairly quick but don't jump the gun - wait until you have roots.

Mike