comfrey

Yes, you can.

make it a big pot, and deep    If you want to use it for topping up the compost, pity you can't grow it in the garden as the good thing about comfrey is that it's roots go deep and bring up minerals into its leaves that other plants can't normally access which is why it is good for the compost heap.

Comfrey can grow into big plants (easily 3 feet +) - if you don't grow a sterile cultivar such as Bocking 14 then yes, the seeds spread everywhere and can then become a problem weed.  The flowers are fantastic bee attractors but you'll need to cut it back regularly before the seeds set.

I get about 3 flushes + of flowers in a year starting in late spring - I think when you chop it back, it sort of resets the clock (like dead heading flowers on roses) and the desire to produce seed for the next generation.  I suppose it would probably flower more frequently if I didn't hack the whole plant back almost to soil surface level and needs time to recover.

As far as I know, I can't get enough of the stuff, but Bocking it certainly ain't...;0)

I have a permanent bed of comfrey on my plot with about 10 plants - and it's nowhere near  enough but I don't want it to look like I'm cultivating weeds LOL.  I use the trimmings as a mulch over summer and after a day or two on the soil surface it withers to nothing.

We grab some leaves, and just poke them into a 2 gal conatiner, top up with rainwater and leave a fortnight.

We either water it straight on to plants that need a lot of potash, or mix it in with nettle tea and 6X liquid.

Smells to high heaven...