How do i take cuttings off the comfrey

I received a load of Comfrey root cuttings a few weeks ago and potted them up but nothing has come up yet....how long does it take usually? Also, there is quite a lot of wild comfrey growing near my place, is that good to pick and make into a feed for my veg? It has white flowers rather than the deep purple my allotment plant has and narrower leaves.

Sorry should have posted a pic of what the wild comfrey growing near me looks like now 

The deep purple flowers are Blocking 14 plants, but GrannieAnnie is right, all is good for feed. 

Blocking 14 has the advantage of being non-invasive.  I bought a plant from work just because I liked the different colour of the flower.

The leaf cuttings, I should have said, take 2-3 weeks and you need to change the water daily and chuck any leaves that look like they are rotting.  Very easy though.  Ended up with a tumbler full of rooted leaves and very few failures