When the trees shed their leaves in the fall, raking up the leaves and using them as beneficial mulch is a simple and natural way to have a lush garden next spring. One way to speed up the process is to shred the leaves, usually by running a lawnmower over them. But avoid doing this for leaves that come from walnut and eucalyptus trees. These leaves have a natural substance that acts like an herbicide. If you place these leaves in the garden, they will inhibit plant growth. Instead, compost them so they naturally decompose.