Hi,

No answers to your question directly.
But where is your willow coming from?
Also there is a guy who sells willow professionally based at Ragmans
lane.
He might help.
URL: http://www.thewillowbank.com/catalogue_2005.html

I guess the only bit I can add is that willow is a mat rooter, so will
probably end up getting to the stone wall eventually.

I am up for willow planting as well.
I would like to grow it as a polyculture though, with a living mulch or
intercrop to avoid the use of plastic mulch etc.

Nice one.
N*