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!

It will probably help to get better responses if you tell us your climate area. Many places have had a pretty hot summer, so it may be that the seedlings died of drought. Although tough, they may need some help getting established.