Comfrey Sterile?

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 our Bocking varieties of comfrey do not set seed even though they are not that far removed from the ordinary Russian comfrey plants.