First off, if you are thinking about planting a Willow tree in your yard, don't do it! Willow trees can grow into magnificent and beautiful trees, but their root systems are extremely aggressive, searching out sources of water wherever they can. I had a neighbor several years ago that was having problems with his sump pump drain line in the back yard. When I dug it up, the 4 inch drain line was packed full of roots. I advised the homeowner to cut down the two Willow trees that he planted or he would continue to have problems. Another neighbor last year actually had roots from a Willow tree in his sump pit! It was a mess as you can see by the pictures and I was honestly surprised to see that the sump pump was actually working.
As far as mulching is concerned, this is something that makes our yards look beautiful, but sometimes people get carried away and cover up the PVC sewer clean-outs. Please be careful not to cover these clean-outs as they are the access to clean your main sewer line going out to the street or septic system. If you have to cover the clean-out for whatever reason, try and mark this in some form or fashion so the plumber that has to clean out the sewer line will be able to locate this.