Grass clippings make a fine free mulch, but don’t use them fresh from the mower. Still green, they give off too much heat as they begin to decompose and can burn desirable plants. Instead, spread the clippings in a thin layer in the sun and let them turn slightly brown before placing them around shrubs or flowers. Or toss them on the compost pile. Never use clippings from lawns that have been treated with herbicides, though.