Mulch is used to cover the soil surface around plants. It can be organic materials like garden bark chips, straw, coarse compost; or inorganic materials, such as gravel, rocks or permeable landscape fabric (not sheet plastic). Porous mulch allows water and oxygen to pass through to plant roots, while providing a layer of insulation.
Xeriscapes use mulch to reduce evaporation from the soil surface, to moderate soil temperature, to limit weed growth, and to reduce erosion. Mulch can also be used to create paths through your xeriscape. Different mulch materials may be suitable for various uses in xeriscapes. Even seashells and sea glass can be used as mulch.
Organic mulches are the best choice because they add nutrients to the soil as they gradually decompose. They also reflect less heat than inorganic mulch. However, they do not work as well on steep slopes or in a drainage path because they can wash away with heavy rains. Lighter organic mulch can also be blown away in windy areas.
Organic mulch can be shredded hardwood, garden bark chips, pine needles, leaves, straw, nut shells, or coarse compost. Some organic mulch occurs naturally from leaves and debris. You can also purchase bagged mulch from nurseries or garden supply stores. Many communities provide bark chip mulch at no cost to their residents.
As organic mulch slowly incorporates with the soil, it will need to be re-applied, “top-dressed”, from time to time. It should be applied at least 4 inches deep. Garden bark chips are a good choice for a new xeriscape. As the new plants grow and spread, they will cover the mulched area.
Inorganic mulch can be placed directly on the soil surface or on top of a porous landscape fabric. It should be applied at least 2 inches deep. Sometimes gravel will scatter, but inorganic mulches rarely need to be replaced. They also work well in windy areas. Japanese gardens make very stylized use of gravel mulch.
Inorganic mulches include slate, stones, brick chips, gravel, rocks and rubber “bark” (from recycled tires). Some plants native to very well drained soils prefer gravel mulch. It is recommended that inorganic mulch NOT be placed near the house on the west or south side. In these sunny spots, it tends retain and radiate heat warming the house, which may increase home cooling bills. Rock mulch can become hot and injure some plants. To prevent that, it should be placed in areas that are shaded by xeriscape plants, which provide environmental cooling.
When the gravel layer is less than 3 inches thick, porous landscape fabrics are useful to control weeds. Do not use solid plastic sheets, as they keep air and water from reaching the soil. Impermeable plastic can be used for areas where you want to keep the soil dry. For example, you might want to use it next to the house to channel water to another area.
Large landscape rocks also help capture and retain moisture in a xeriscape garden. If you turn over a big rock, you can see the damp soil underneath. Plants near the rock can benefit from that moisture.
Whether your xeriscape uses garden bark chips or gravel, mulch needs to be several inches thick to be effective. It may need to be replenished as time passes. There should be no bare spots.
In the video below, Master Gardener Linda Poe shows the variety of mulches used in the Water-Wise Garden at Rio Rancho, NM including pine chips, pecan hulls, and crusher fines. This xeriscape garden was designed, installed, and is maintained by the Sandoval County Master Gardeners.
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