[ConfusedGardener]

I potted up 5 pieces of Brocking #14 around this time last year, and they eventually went in the ground around Aug/Sept. They are now almost 2 feet tall, and about to flower for the first time.

[BabbyAnn]

are you asking how to do it, or what to do with it?

Comfrey is very hardy so brutally cutting it back to almost ground level won't usually kill an established plant - just give it time to recover between harvesting to replenish food in the root system for future cropping.  I usually do about 3 lots during the year (1) about now for layering leaves in potato trenches (2) one about late June/early July as a mulch round my winter squashes (3) about October when new growth is  unlikely but the leaves haven't been hit with a frost and these go into the compost bin.  Just be aware that you use the leaf portion and not the stem (or flowers) especially near the base where it may take root.  Your comfrey plant will have a deep root system and  be able to take up nutrients where other plants can't reach, but it will still need feeding so give it a spring mulch of compost for some nitrogen for healthy leaf growth.

An alternate which I don't do is to make comfrey tea - soaking the leaves in a barrel of water until it rots - but be prepared for an awful stench though 

[ConfusedGardener]

Thanks for the reply Babbyann,

I suppose what I'm asking is - is it established enough to start using/propagating?

I've used comfrey to make tea before, but this is the first time I'll be able to use some that I've grown myself.

[gavinjconway]

Cut them flat and they will grow again in a few weeks.. I take about 3-4 cuttings a year off mine.

[bravemurphy]

Myself I would leave for a season to get nice and established and then next season cut them.

also take lots of root cuttings for even more 

[JohnB47]

I would harvest the leaves as if they were well established plants - they are very hardy once they get going (as yours have).

[gavinjconway]

I would harvest the leaves as if they were well established plants - they are very hardy once they get going (as yours have).

Yup - totally agree

[Growster...]

Our comfrey bed, (definitely not Bocking), is now about 3' x 9', and growing fast...

We usually make the tea, as you can just chuck it all in a barrel of water and dunk the watering can each time you want to feed anything! Someone clever here had a super piece of kit, made from a 4" plastic drain pipe screwed to the shed wall, into which she just poked a few leaves every now and then, and collected the brown gunk which eventually drips out at the bottom!

I'm still trying to find time to make something like this, as it appears to be the least pongy way to get the best from this amazing plant!