Planting comfrey/mustard

Kings seeds sell mustard which I've just planted - takes 4 weeks apparently to grow and can be dug in before flowering - link here:-
http://www.kingsseeds.com/kolist/1/VEGETABLES/V-G/GREEN+MANURES

You'll have to be quick though!

Comfrey is a permanent plant which you crop regularly to add to your compost heap or to soak to create a liquid feed.  If this is what you are after, try to get the Bocking 14 variety as it is much less invasive.

I think you are actually asking about the green manures - of which mustard is one.  These are plants which are grown and dug in before seeding.

There have been a lot of posts recently about green manures, including mustard, might be worth a browse back or a search to see what else is possible.

If you have club root on your plot, do not use mustard as it is part of the brassica family.

Bocking 14 won't wonder about as it is "sterile" -unless you chop up its roots.
Fantastic plant that not only produces a excellent liquid feed but also attracts loads of bees.
I grow loads of it.