Not all soil covers make good mulches
Mulch is any organic substance used as a top cover to soil that enhances the rooting environment of plants. In this context, the term organic refers to substances that readily decompose. While they are often used as soil coverings, materials such as gravel, shell, volcanic rock, limestone, and granite screenings are not actually mulch. These ground coverings do not provide many of the benefits of mulch, such as holding moisture, moderating temperatures, and providing tilth for the soil. Since they retain cold or heat longer, they may keep soil too hot or too cold to benefit plants.
Organic mulches come in many forms, colors, and textures. Popular mulches vary by region and, for the most part, are related to what’s locally available. Most mulches sold commercially are by-products of forest harvesting. Much of the free mulch given out by cities and towns comes from utility-line clearing and tree trimming; some is made from recycled brush and Christmas trees.
The most common mulches are chipped or shredded bark, chipped tree limbs, or shredded whole trees that are too small for commercial harvest. Trees often used for mulches include southern pines, cypresses, northern hardwoods, and eucalyptus. Other common mulches include cocoa-bean hulls, pine straw, and leaf mold.
Recently, recycled wood pallets and woody construction debris have been ground up, dyed or stained, and marketed as mulch. I would be concerned, though, about the potential impact of dyes or stains leaching into the soil.
Many composted soil amendments, such as mushroom compost, manure, and grass clippings, are also used as mulch. While beneficial in nourishing soil, these materials decompose quickly. Thus, they are not as effective in moderating soil temperatures or retaining moisture unless they are reapplied frequently. Avoid using thick layers of grass clippings as mulch, since they can become a barrier, sealing the soil surface.
Choosing mulch involves both aesthetic and practical issues. Packaged materials are no better than bulk mulches from local sources, though they may be easier to move around. Mulches from local sources are sometimes thought to potentially transmit diseases and insects, but I’m not aware of any confirmed reports of plant diseases transmitted by mulch. Again, the best precaution is to use mulch that has begun to decompose. Both bagged mulches and those from municipal sources have usually begun to break down by the time they are available. As mulch ages, it becomes darker in color.
There are three common textures of mulch: shredded, tub-ground, and chipped. Shredded mulch is usually composed of thin strips of varying lengths of bark. Cypress and eucalyptus are the most common shredded mulches. Tub-ground mulch combines very fine to medium-sized particles. Because of the large amount of finely ground material, tub-ground hardwood mulch often has a dark, rich appearance. Chipped mulches are usually coarsetextured and commonly available, especially as free community mulch. While some mulch products specify the exact ingredients, others do not. I’m leery of bagged mulch that is labeled something vague such as “midwestern mulch,” since it may contain recycled construction materials.
As mulch decomposes, it releases either acidic or alkaline substances into the soil, depending on the mulch. This only matters if you’re trying to grow acid-loving plants like azaleas and rhododendrons. For these plants, use a mulch that releases acid, such as pine straw, pine bark (which is also high in aluminum), true cypress, and some species of eucalyptus. In contrast, hardwood mulches tend to become alkaline and are good for almost any plant that doesn’t require an acidic environment.