Bocking 4 vs Bocking 14 Comfrey Sterility

I'd like to know whether Bocking 4 comfrey is sterile, just

like Bocking 14.

Also, I'm considering planting both for separate purposes - Bocking 4

for chickens and

Bocking 14 for compost, and would like to know if they'll

cross-pollinate and set viable

seed if I plant them too close to each other.

Both of these varieties have proven to be completely sterile in our gardens. They can only be propagated by cuttings of roots or in spring - of shoots.