I have heard that oak leaves, in particular, have natural germination-inhibitors before they break down, and that's been my experience as well. I't hard to get enough of them. I used to have a huge leaf pile and would raid it in the spring to use as mulch. One can sort of "tile" with slabs of damp, partially-decomposed hardwood leaves. It is thicker and stays in place better than fresh or mulch-chopped leaves. Amazingly effective for nutrients and week-suppression if one does it every spring. Some weed seeds will be germinating in the fall, but they aren't too hard to pull in that situation.
