Sterile Comfrey?

I bought some 'sterile' comfrey a while back - it's doing well in pots until the beetroot have been pulled from the future comfrey patch.

One of the comfrey plants has flowered (rather pretty and purple). Does this mean that it's not sterile? Does sterile comfrey not flower, or does it flower but produce no seeds/non viable seeds?

Thanks in advance,

SS

Great stuff - Ta - SS