Grass clippings should be left on the lawn when mowing. “Grass-cycling” is natural and an environmentally beneficial practice. Grass clippings are about 90 percent water by weight. Because they are high in protein they should be left on the lawn to decompose and add nutrients to the soil. Grass-cycling also reduces waste and conserves landfill space. It is only necessary to remove lawn clippings if they are long and will smother the lawn.