Growing Comfrey Indoors - In Cups - Help!

I live in Zone 3 where we have 110 days of growing season.
As an experiment, I dug out some comfrey plants so that I could divide them and grow them over the winter under grow lights.
I started this process on November 13, 2018

I put the comfrey roots in very nice soil with compost, peat moss etc... it's a nice growing environment.
I have the grow lights on 12 hours per day.

I am growing 30 root pieces hopefully into plants.. I want to use comfrey in the yard to help build the soil with chop and drop.

The Problem:
with a few of the comfrey plants they were growing nice leaves.  Now the leaves are dying.
I took one of the plants apart to see what was going on.  I noticed that the root piece hadn't grown any roots yet.. it was just growing the leaves but no roots..

I have watched many videos where people took root pieces and grew viable plants from them.
I don't want to waste these root pieces as I could have just left them in the ground to grow, and they were doing fine...

Is there a way to stimulate root growth?  
If these initial leaves die, can I revive the root and have it grow some more leaves and start roots?

Thanks,

To get a comfrey root to grow a complete new plant you don't want to use a rich soil and you want a fairly good sized container (6" pot at least).
I use sharp sand to start root sections, it allows me to prevent damping off since the water drains away quickly.
Check the distance from the light source to the plant leaves, if to close you will burn the tender new leaves and that will kill the root section.

Redhawk

Thanks for your reply..
how close to the grow lights would you suggest?
and... I have sand... it's currently frozen in the shed, but i can bring it out and thaw it...

should i replant the plants in pure sand?  or sand / dirt combo?

I would suggest trying 25-50% good soil & the rest sand. Mixed together. If the grow light burns the back of your hand it is too close.

Ok... thanks.. I will try that sand/soil mixture...
as far as the lights go... they aren't hot at all.... they are 4ft T8's  … they are cool to the touch...

I use 3 1/8" deep cells (1801 tray inserts) filled with Pro-Mix BX but a 3:1 perlite to peat moss mix would work just as well.  I use a tall humidity dome and keep them really well watered.  With the perlite, they won't be waterlogged and rot. I insert the cuttings vertically with the top exposed. The first growth I see is usually callusing which then produces leaves. Sometimes I see green growth from the other buried end. My grow lights are just above the humidity dome.

To stimulate root growth, I water with a liquid fertilizer with a high phosphorous number 9-45-9, micronutrients and seaweed.  I also use it as a foliar feed.

I get around 100% of my root cutting to turn into plants.

I want really good root development (it will be fibrous rather than woody) to I let the plants get pot bound.  If the top growth gets to be a jungle, I'll cut back the leaves to an inch or two. They resprout very quickly.

When I plant out, I wound the roots and correct the pot bound growth by slicing a cross about 1/2" deep across the bottom of the root mass.

Monty Loree wrote:
The Problem:
with a few of the comfrey plants they were growing nice leaves.  Now the leaves are dying.
I took one of the plants apart to see what was going on.  I noticed that the root piece hadn't grown any roots yet.. it was just growing the leaves but no roots..

How far from T8s and what T8s? I put 1" cuttings into ~1" starter pots and put them 6-12" under a 4x4' T5 (216w) and then transplanted them to plastic picnic glasses when larger.  18/20 grew and are doing well.  I know which ones didn't and they were not optimal cuttings.  
More later...

Edit:
Yeah, I don't know exactly how far the young sprouts were from the lights, but I know they were close enough to mostly have >90 degree angle from one edge to the other of the T5 fixture (probably closer to 120-160 at the center), so moving them closer wouldn't have provided much more light, just less dispersed.  I did make sure the light was high though.  Those lights get a lot warmer, but I don't think the plants ever got so warm that new sprouts would get too hot.  

However, putting them under higher light means they get drier quicker, so I made sure they were kept moist.  I feel that if they were kept cooler, dryer, or with less light they may not have done so well.
I just used potting soil.  I grew them with other larger comfrey cuttings I purchased (a blue comfrey) under the same light, as well as sprouted seeds, and they more or less all do well.  The main sources of dying leaves I believe has been under-watering.  Especially once they get leaves, they can really regulate their own root moisture (to an extent).

I have tried letting comfreys grow a little in a partially sunny window, and they just seem to barely hang on, sometimes eventually dying.  
They really do like light, so in my dealing with a limited budget, I'm trying cycling them through sitting in windows, kept moist and cool and not thriving, until they get their own turns under the bright T5s. I also have a 3x5W LED light where I can focus each LED node onto just one small plant, to supplement the larger fixture. Some plants I have quasi-moist in the fridge, and they seem to sort of do nothing.  If they start dying, I'll cycle them into the bright light treatment.  Hopefully I'll keep them alive until Spring!  I'd really like to be able to get them as dormant as possible where I can warm them up and plant in the Spring.

FYI, I went into more detail of this attempt at https://permies.com/t/40/65490/Growing-Comfrey  
It's my first comment on thread, page 2, posted 3 days ago with the previous comment 1 year ago, if that helps.

Comfrey is cold hardy plant. Pribably not necessary to bring indoors

I received some crown segments with roots, I planted them in some extra nursery pots that I had.

The soil mixture I utilized was mostly coir with perlite to help with drainage. There also is worm castings and some processed poultry manure for a little bit of nutrients. I've added some scrap wool in small rounds deep in the soil for pockets of moisture/degrade into nutrients. For science of course.

We will see how they do, I already have leaves bursting up.

Our winters are relatively mild and have found that root cuttings survive fine grown outside all winter.

In autumn I pot up 6" sections of roots that are at least 2" diameter and leave a couple of pots where I'd like to establish a new clump a comfrey.

I leave them there for at least two seasons to allow the roots to grow through the drainage holes at the bottom of the pot. Try to lift the pot and if it's stuck, the roots have grown into the ground.

Use a spade or sharp knife to slice across the bottom of the pot and start another clump in another spot.

I think people are overthinking this whole Comfrey growing process. It may be just the variety I have, but it is pretty easy to grow indoors:
- use a clear plastic container
- fill it half full with store-bought compost ( you can use garden soil but I don't because there seems to be bugs originating from it)
- put in a layer of root cuttings
- cover with some more compost
- keep it moist/put a lid on

The clear plastic helps you see if it is still moist

M