dirk hoorelbeke
Omono
I've tried with washer, cd, wire ... couple of times (acer, ulmus, cotoneaster, carpinus, fagus). Yes they thicken, yes there are some roots. Not very successful to have an even spread. The thickening might be a good base to start on a girdling, but in my climate not sufficient results to build roots. I've seen very good results online so it is possible, but i never get there (not in ground, not in pot).I put 3 seedling elms through a washer this spring, they grew ok....
I have dug down to the washer and removed high roots 3 Times, and still only have one root on the washer, which may be too big by next year.