Mulch is any material laid on the surface of the soil to improve growing conditions. Mulch insulates plant roots from both heat and cold, reducing moisture loss from the soil by evaporation, feeding beneficial soil microorganisms that enhance plant growth, and improving soil health. Because organic mulches were once alive, they contain essential nutrients, minerals, and energy that nourish the soil ecosystem as they decompose. An important consideration when using mulches is to skip using landscape fabric, which prevents both the rich organic matter created by decomposition from working its way back into the soil and rainwater from soaking into the soil.
Below are the basic types of locally available organic mulches that we recommend. Remember to install to a depth of 3”-4” over bare dirt.
Wood chips: Wood chips are chipped pieces of trees. This is a long-lasting mulch that decomposes slowly over bare dirt, retains moisture, and prevents weeds. Most wood mulches available on the market are chipped flat and consistent in shape in size. These are great in areas that receive foot traffic as they compact down to a firm surface.
In areas of higher wind exposure or periodic inundation with water, use a shredded wood mulch that is more irregularly shaped and sized as the pieces will knit together better. This type of mulch can be recycled yard waste and tree trimmings as well as commercially available, locally processed wood products.
Pecan Shells: This long-lasting mulch has a nice color and good texture. It is very slow to break down into the soil. However, it attracts birds and rodents and blows in the wind unless partially cultivated into the soil. It also tends to mat together, creating a crust over the top of bare dirt.
Straw: Straw breaks down quickly so it is most useful for mulching vegetable beds where it is renewed as crops are replanted. It is a good general mulch for winter protection and can be used on paths between vegetable rows. Keep in mind that it may carry weed seed.
Pine Needles: Pine needles are a very good mulch. They are light, airy, and attractive but can be a fire hazard. These needles are safe to use as mulch around most plants, especially perennials, in your garden. Because pine needles contain high levels of resins, they repel water and decompose much more slowly than leaves from most deciduous trees. Because of their shape, they don’t become moist or compost rapidly. This makes them especially useful in areas where you will not be digging frequently. The fragrance of pine needles on sunny days is also an added benefit when they are used as garden mulch. No need to be concerned that pine needles are too acidic for use in our gardens. The added acidity, which happens slowly, is beneficial to our soils. They are very slow to compost, so they are often a better mulch than compostable material.
Bark: Large 2”-4” bark pieces are not recommended because they take too long to break down. The mulch made from bark tends to be lighter and can tend to migrate more easily. The bark is naturally water-resistant, so it has minimal value in feeding soil microorganisms and improving soil health.
Crushed Stone, Crusher Fines, or Decomposed Granite: Crusher fines come in a variety of aggregate colors. They are recommended for use as pathways and patios or as mulch under plantings for heat-loving, desert-accent plants.
Gravel Rock and Cobble: While you see these used throughout the city in landscapes, they are best used to prevent erosion on steep slopes or in swale areas. They are difficult to maintain without regular weeding or pre-emergent treatments.
Useful tips: Mulch typically packs down over time. Use a 3”-4” layer of mulch, which is about one yard of mulch per 100 square feet. “Spend an hour deepening the organic mulch and save several hours of weeding.” By Judith Phillips