Comfrey Tractor

So I tried the comfrey tractor idea in two milk crates under my pear tree.
I was hopping the resulting comfrey patch would crowd out less useful undergrowth.
What I got instead is two nice happy comfrey plants with no sign of them rooting in the soil below their planters.
I never water them, but I guess the junk in thier planters holds enough  water and nutrients that they "feel no need" to root deeper.
I'm gonna move these plants to the yarden and attempt to use them in direct sunlight.
The pear tree will be planted with comfrey the old fashioned way- kick at the dirt, tos root fragment in resulting "hole" , stomp , come back in a month to a full grown plant...

Anyway, any ideas to get my comfrey tractors working better?
Should I maybe repot in sand or something?

I can think of two ideas:
1. Actually dig the milk crates into the dirt by 6" or so, to encourage the roots to go out sideways as well as down.
2. Plant 4 narrow plant pots beside the crate and put the water in them so it flows out the bottom holes instead of watering the crate itself. Hopefully the comfrey roots will head downward seeking the water from the pots.

I've never been sold on the comfrey tractor plan. I think it should be made to work, given adequate time in one spot, but it doesn't seem worth the investment of effort.

I can stick a spade through an established crown and lift a huge clump of roots in two minutes. 5 minutes work and that crown is 20+ root fragments. Provided the soil is warm and moist they seem to take really well for me, with minimal effort.

Helps a bit of the soil is tilled to supress grass initially, but I have managed to get them going with just a spade slit to lever the soil open.

EDIT: I want to clarify that I have actually tried the tractor method as well. We had a pot of well established comfrey parked in a spot for well over 18 months. I was hoping for some subsequent growth, but nothing appeared.

I’m not familiar with the comfrey tractor method of propagating comfrey but what I find really simple is to put good chunky cuttings in a small pot and leave where I want them to grow.

They are left to fend for themselves, the spot is along a wooden pallet fence that runs N-S. it’s relatively slow, at least two growing seasons but by the time the leaves have sprouted twice, they die back in winter in our climate - snowing overnight this evening.  

I just cut through the bottom of the pot, slicing through the roots that have grown into the soil below the pot and move to the next spot that I want to get them established.  Our chickens love brassicas, sorrel and comfrey, surprisingly they really like the stems, the concentration of allantoin is higher than in the leaves, only bettered in the roots.  

Got to love comfrey. I make a salve with dehydrated comfrey leaf and root, steeped in evoo and melted beeswax that is used on everything from our lips, hands, stings, burns etc

Your method of moving a pot is pretty much exactly what is meant by a "comfrey tractor". It just seems to work well for some people but not others.

Seems my containers may be too large.
I will try with smaller containers, coffee can sized.
The main advantage to containers might be the instant mulching of competitors.

I was thinking about adding comfrey under my black berry bushes,  good idea or too competitive?

I have way to much comfrey. When we built our new home 5 years ago the excavation moved comfrey bits everywhere.  I have/have had them in my blackberries and in my flower beds.  I eradicate them in those spots because here they are very very competitive.  I would get 5hem out of any competition.

If you can!