Why and How Much To Mulch

Mulch has an underserving bum rap these days. I feel compelled to come to its defense. There is much more than meets the eye about mulch.

Here are the benefits of mulching:

  1. Mulch retains water – Mulching significantly reduces moisture evaporation from the soil. Studies show that a two-inch layer of mulch cuts the water use in a planting area by 20 percent. For California’s climate, especially in the face of the continuing trend of drought, water savings alone is enough a reason to mulch.
  2. It moderates soil temperature extremes and protects plant roots – Mulch insulates and thus alleviates the soil from becoming parched in hot weather. A two-inch layer of mulch can lower the soil temperature of the top four inches of soil about 10 degrees Fahrenheit. And when we experience occasional freezing days in the winter, mulch helps protect the frost-tender plants from damage.
  3. Mulch reduces erosion and mitigates storm water run-off – the bare soil on even very gentle slopes erodes easily from the impact of rainfall that can cause the loss of precious topsoil. The topsoil could also be blown by the wind and carried away. Both the velocity and the volume of run-off water from a mulched site are reduced, resulting in an improved infiltration rate.
  4. Mulching is a very effective weed control measure. In annual beds especially, mulch can reduce weeds by as much as 90 percent. Compared to herbicides, mulching is not only environmentally friendly it also saves labor cost. Laying mulch on the ground or adding mulch periodically can be done just about by anyone, while spraying herbicide must be performed by certified personnel. In addition, herbicide-use needs to be applied more frequently then mulch.
  5. Organic mulch improves the soil structure – its physical characteristics – by enhancing porosity as it works its way into the soil aggregates; mulch prevents soil from crusting and compaction. Organic mulch also supports microbial activities and beneficial organisms that benefit the soil structure.
  6. Organic mulch adds nutrients to the soil as it decomposes and amends the chemical and biological makeup of the soil.
  7. Mulch reduces waste – Plant debris accounts for approximately 7 to 8 percent of what is hauled away to the dump. Landfill volume is reduced by using plant trimmings and chipped wood as mulch.
  8. Uniform sized, textured and colored mulch can enhance the look of a bare site. As wood chips age to a silver gray color, they provide a natural look. Mulch is also used to cover drip irrigation lines and aides maintenance of drip system.

Types of mulch

Mulch can be any material that is spread evenly over the surface of the soil to enhance the growth of plants and appearance. There are two basic types of mulch: organic and inorganic. Inorganic mulch includes materials like rocks and gravel, plastic sheeting, and rubber. Rocks and gravel will absorb heat and warm the soil and so are generally not appropriate for our climate in landscape situations. Plastic sheeting is usually used in controlled vegetable gardens.

Organic mulch used in landscaping can include a multitude of recycled materials:

  1. Chipped or shredded wood wastes from used lumber or wood pallets. These break down very slowly and thus last longer than other types of mulch, but they add little nutrients to the soil. These are best used for pathways.
  2. Wood chips from tree removal or pruning and bark make excellent mulch material. They are usually uniform in size and appearance; and they stay in place and last a medium long period. Rake and replenish at least every other year. Wood chips age to a silvery grey color, but bark tends to keep its color.
  3. Gorilla hair is made from shredded redwood bark and is a tufted and fibrous mulch option. It stays well in place so is suited on slopes. It decomposes slowly and still adds nutrients to the soil. But a thick mat of gorilla hair may prevent rain or irrigation water to seep through and deprive the soil of moisture. A thin layer of no more than 1-1/2-inches of gorilla hair should be used, except at edges.
  4. Mulch made from mixed plant debris is from the above shredded materials plus brush and other plant trimmings. It is finer in texture than pure wood chip or bark. It settles faster and needs to be replenished at least every year or when the thickness is less than 2-3 inches. It can provide a broad range of nutrients as it decomposes into the soil.
  5. Pine needles do not pack down and they resist decomposition. Pine needles do not absorb water like mixed plant mulch but let water trickle through. We do not use pine needles near structures, since dry pine needles can be flammable.
  6. Compost from plant and other organic matter can be used as mulch that provides valuable nutrients and improves soil structure. As it is not effective as weed control, we often add a layer of wood chip on top of compost to suppress weeds.
  7. Leaves are good mulch for providing nutrients and trace elements, so leaves should be left where they fall; leaf drop is nature’s way of returning nutrients and organic matter to the soil. Of course they should be raked away from pavements and from storm drain inlets.
  8. “Grasscycling” is the practice of leaving grass clippings on the lawn as mulch, a good practice that provides nitrogen and other nutrients to the lawn, reducing fertilizer and pesticide runoff. Clippings can also be spread over planted areas and rake it lightly to settle into the soil.

Mulch is usually applied in 2- to 4-inch settled thickness. Fine textured (1/2-inch particle size or less) mulch is generally applied in thin layers to avoid impeding air and water. Most professionals specify at least 3 inches of mulch of any coarse mulch material except on poorly drained soils.

This article first appeared in the November 9, 2016 issue of the Rossmoor News, author Changlin Dillingham.

One thought on “Why and How Much To Mulch”

  1. Wow, I had no idea that a 2-inch layer of mulch can cut the need for water in a planting area by 20%! My sister and I are working on building a garden in our home, but we are still beginners at this. I will be sure to remember this when we are actually putting everything together, and find the best mulch for what we are doing.

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