White Lilac Pruning
 

I have a White Lilac which is surrounded by mature rhododendrun bushes and it is reaching a size where I fear its water requirements will compromise the surrounding bushes.

Ultimately, the lilac is expendable, but I would rather retain it and keep it in check. Ideally, I do not want the roots to extend any further and if at all possible, to retreat a bit.

If I seriously prune existing growth and remove a couple of big branches low down and keep the tree to its new dimensions, will I be able to almost arrest future root development?