New here... getting comfrey to thrive

Hello.  I'm very very inexperienced.  

I understand some basics, including the need for nitrogen fixers and dynamic accumulators.

I started ~20 comfrey seeds in a grid planter until they sprouted, then transplanted them into the ground, since I am aware that comfrey requires a lot of below-ground space to acomodate the deep roots.

However, I have only one left alive after the summer.  It must be something I'm doing, since I'm of the understanding that comfrey is a weed that settles in and can spread without much intervention.  Instead, I'm struggling to keep just one of them alive.

Any and all advice is welcome!