comfrey plants

Is it Bocking 14 you have coming ?  It doesn't set seeds and take over like the other varieties.

This should tell you all you need to know:

http://www.allotment-garden.org/vegetable/comfrey/index.php

The answer is always on here somewhere.  From my personal experience my best comfrey is at home in moist/almost boggy soil next to a north facing fence.  It is huge already.  The comfrey bed at the allotment is in a windy spot on free draining soil, it does not struggle but it does not romp away like the back garden one.  Both Bocking14 from the same supplier in Devon.  This year I shall water the comfrey bed at the allotment if needed, (not at the moment!).