The ideal mulches are life based; that is, from living tissue (as opposed to rock or plastic) such as: wood chips, bark, root chippings, pine needles, grain straws, shredded leaves, and so on. Mulch forms an insulation blanket to conserve moisture, protect roots from summer’s heat and winter’s cold. Mulch also helps suppress weeds. Aside of good watering, mulching is the single most important thing you can do to assure garden success. Recommended depth is 2 to 4” on new plantings. Over time, mulch decomposes so refresh it annually by top dressing with another inch or so.