What is mulch? The term mulch means to cover the soil surface with a substance or layer that will help to reduce weed growth and help hold in moisture. Some common mulches are bark, rock, compost, grass, fabric and straw. The best mulches are compost-based and replicate forest litter. Most plants love compost-based surface mulches because this is the normal condition that almost all plants have evolved to expect. No one goes out and cleans up the forest. This leaf litter naturally breaks down and encourages micro organisms that assist in breaking down nutrients that the plants need to survive.