I've been going through a bed that I made 2-3 years ago that is getting weeds & grass. I pull back the mulch, pull up weeds and pull out grass rhizomes, dump my dirty bunny bedding, cover with 2 layers of cardboard, and dump the existing mulch back on (I really need more mulch). It is working well, but takes a lot of time. I also have to do a section a week when it is time to dump the bunny bins and I have a long stretch (at least 50 feet and maybe approaching 100 feet).