Are dead leaves harmful to plants or soil?

Yes, leaving fallen leaves to decompose does return valuable nutrients to the soil, provides habitat for lots of important and valuable insect species over winter, and acts as a natural mulch. The only way to leave the leaves on your lawn is to chop them finely with a mulching mower or a leaf shredder, or shred them in a trash can with a string trimmer, then return them to the lawn. You cannot leave a layer of fallen leaves as-is on your law, unless you want to have to do a lot of lawn repair next year. Layers of leaves block sunlight and trap excess moisture against the lawn, resulting in bare patches come Spring. It’s also a good idea to keep layers of leaves off of beds of Autumn and inter-interest plantings like pansies for the same reason. A thick layer blocks sun and risks disease in wet weather. Rule of thumb: if you can’t see the plants underneath, the leaves are probably going to cause a problem. Yet there are plenty of places where you can leave the leaves. You can leave leaves in wooded areas, on mulched areas, under shrubs and around perennials as long as you think of them like mulch: not built up too thickly (3-4” at most” and not piled up against stems and trunks.