Comfrey Bocking 14 
 
Comfrey really is a super plant.  This particular strain is a sterile hybrid that does not produce viable seeds. What that means is that this variety will not spread.  You can propagate it from root cuttings and flower stalk cuttings, but it will not spread via seed. 
 
Comfrey sends down a deep tap root that pulls up nutrients from the sub soil.  It is naturally high in Nitrogen but also contains other trace minerals such as potassium, silica, magnesium, calcium, silica, and iron. Said to contain 17% in nitrogen with a NPK value of 3-1-4.8. 
 
Makes a great compost tea high in nitrogen to make things grow faster, stronger and healthier.