comfrey

I've just collected a large bag of comfrey and put it in my water butt.
I do this every year to feed tomatos etc. Supposed to be just right when it smells really foul.
Question is as it was in flower and with seed heads on it, will leaving it in the water butt for a couple of weeks kill off the seeds or am i going to have comfrey coming up everywhere next year ?  Not a nice prospect !!

Any thoughts on this would be appreciated.

I'm also told that comfrey mixed with sheep droppings are the very best.
Necter of the gods, so I,m told.

Also comfrey makes a good accelerator for the compost heap, but i guess not when its in seed.