Comfrey, part one. I'll write about various Comfreys as and when I see them coming into flower. Above is Symphytum ibericum on the allotment. Beth Chatto described S. ibericum as "impenetrable weed-cover" and Martin Crawford calls it a "fantastic ground cover". 

 I planted a few root cuttings taken from a patch on a nearby allotment and within a year or two it has fulfilled those criteria.