By Crystal Kim, Red Butte Garden Horticulture Program Coordinator
Mulch is a layer of organic or inorganic material, spread on the surface of the soil, around garden plants. Most plants benefit from some type of mulch. Mulch helps reduce weeds by shading the soil and preventing some weed seeds from germinating. It can add to the aesthetics of a garden. The most important benefits of mulch in a hot, dry climate are reducing soil surface temperatures, improving water infiltration, and retaining soil moisture, thus reducing the amount of water needed to keep plants healthy and thriving.
What is mulch
Mulch products can be classified as either organic or inorganic, depending on the origin of the materials. Organic mulches are typically derived from plant materials, such as wood chips, shredded bark, pine needles, grass clippings or shredded leaves. Compost can be used as mulch when spread on the soil surface rather than incorporated into the soil. Organic mulches reduce erosion and benefit soil life, which in turn improves soil structure and plant health. They add organic matter, water-holding capacity and nutrients to the soil as they biodegrade. However, for low water plants adapted to low-nutrient soils, the extra nutrients may be a drawback.
Inorganic mulches consist of nonliving materials such as gravel, stone and their byproducts. Since they do not retain excess moisture at the soil surface, they are suitable for low water and xeric plants that dislike moisture near their crowns. These mulches absorb and reflect heat, which can be an advantage for water wise, heat-loving plants. Inorganic materials do not add nutrients or organic matter to the soil, which may or may not be a benefit depending on the needs of the plant.
Another product is rubber mulch, which is made from recycled, ground tires. While using recycled materials is encouraged, note that rubber mulch is highly flammable, and its toxicity is still being studied. Also, there is some concern with heavy metals, including zinc, leaching into the soil as it decomposes.
Landscape fabric and plastic sheeting are often used as weed barriers. Plastic sheeting is never recommended because it forms an impenetrable barrier to water and air, and eventually breaks down into persistent and potentially harmful debris. Landscape fabric, on the other hand, is permeable to air, water, and nutrients, and is appropriate in certain situations to keep inorganic mulches like rock separate from the soil. However, it is costly, has short-lived effectiveness and restricts habitat for beneficial insects. It is also aesthetically unappealing and is typically covered by a layer of mulch to improve its appearance. A mulch layer, if used, will eventually provide a good growing medium for weed seeds that move in later.
What mulch you choose to use depends on your budget and personal taste in materials, in addition to the needs of plants in your landscape. Gravel and stone-mulched beds can be difficult to weed. Also, when growing short-lived perennials or annuals which are frequently removed or replanted, gravel mulch has a tendency to become incorporated into the soil, which can be a maintenance issue since it doesn’t decompose.
How to use mulch
Mulch should be applied generously, using one to three inches around perennials and annuals, and three to four inches around trees and shrubs. Do not pile mulch around the base of plants. Organic mulch may need to be replaced or added to periodically to maintain the proper depth, depending on how quickly it breaks down.
If a high-carbon, low-nitrogen material, such as wood chips, is used, nitrogen in the soil often becomes depleted, thus temporarily depriving plants of this essential nutrient. This is due to soil organisms requiring nitrogen in the decomposition process. It appears that the depletion mainly occurs at the soil surface, so plants with deeper, more extensive root systems such as trees and shrubs should not be impacted. Annuals and perennials are more likely to be impacted, so adding nitrogen fertilizer or spreading a thin layer of compost before applying wood chips will help.
Trees have particular needs when it comes mulch. Organic mulches, especially those that are coarse and woody, are best. Organic mulch benefits shallow feeder roots by protecting the soil from compaction, keeping the soil shaded and cool, and adding organic matter and nutrients to the soil. Mulch should never be mounded around the trunk. Keep mulch pulled back a few inches from the trunk to prevent disease and rodent damage. Create a mulch bed several feet in diameter around each tree, to keep plants such as turf from growing directly underneath and competing for water and nutrients. Keeping grass well away from tree trunks also reduces exposure to pesticides and fertilizers applied to turf, as well as damage caused by mowers and string trimmers to the bark and living cambium just below the bark. Inorganic mulches such as rock, gravel or weed barrier fabrics are not recommended.
In addition to plant health and aesthetics, consider the presence of native pollinators in your garden. Many native bees are solitary and ground-dwelling, so they need easy access to the soil. You can provide habitat for them by leaving some areas of soil mulch-free, such as the back of planting beds where it’s not as noticeable. Native bees are important pollinators so encouraging their success is a win-win for everyone.
Sources of mulch
There are many sources for purchasing mulch. Garden centers offer a variety of products to choose from, which can be purchased in bulk or by the bag.
Many landfills have green waste recycling programs. Consider your local landfill when looking for an inexpensive and sustainable source of landscape mulch and compost.