Mulching is excellent for soil improvement. It suppresses weeds and enhances soil structure and fertility. Mulch mimics the natural environment found in forests where leaves and branches blanket the soil surface, replenishing nutrients as they decompose and creating an ideal environment for root growth.
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Mulching helps with the conservation of moisture by increasing water infiltration and slowing of evaporation. Mulch also acts as a barrier that blocks sunlight for weeds to germinate and grow.
Types of Mulches – There are two types of mulches, organic and inorganic. Organic mulches include wood chips, hardwood and softwood bark, pine needles, straw, grass clippings, seed hulls, etc. Inorganic mulches include gravel and rock. The various organic mulches decompose at different rates and require periodic reapplication. Inorganic mulches, such as gravel, stone chips, river rock, and rubber, do not provide the same benefits as organic mulches as they do not decompose.
The Negative Impacts of Black Plastic and Landscape Fabric – A common practice is to apply mulch over black plastic (polyethylene) and woven plastic weed barrier fabrics (polypropylene). Black plastic is impermeable therefore no oxygen exchange can occur to the soil. Lack of oxygen to the roots and soil microbes significantly reduces plant growth. Black plastic also prevents water penetration.
Weed Barriers – Costly & Ineffective
Woven weed barrier fabrics initially allow some minor oxygen and water exchange to the soil, but eventually become clogged and create the same issues as plastic. Weeds easily germinate on top of the fabric and root into or through it. Both plastic and woven plastic fabrics disrupt the life cycles of many pollinators and other soil invertebrates. Fabrics and plastic can be good choices for large-scale vegetable production where regular maintenance and replacement is easily performed.
The selection of a mulch depends on its intended use. If the main objective is soil improvement, organic mulch that gradually breaks down is a good choice. If the area is used primarily for annual flowers, it often is more practical to use a temporary organic mulch such as composted leaves or grass clippings that can be turned under each fall. Wood chips and inorganic mulches like pea gravel are a good choice for landscape plants such as trees, shrubs, or perennials.
Pea Gravel Instead of Crush or Rocks – Rocky materials used for mulch should be smaller than a half-inch in diameter for the benefits of water conservation and weed suppression. Pea gravel has been shown to improve water infiltration into the soil, especially after heavy down pours. Larger stone sizes do not function well as mulches.
Most mulches, including wood chips and pea gravel can be applied to a depth of 2 to 4 inches. Mulch that is too deep can weaken plants and prevent water and air from reaching the soil. Bark or wood chip mulches mixed into the soil may starve plants of nitrogen. If this occurs, a nitrogen fertilizer can compensate as needed.
For tree health, all mulch materials must be kept away from the trunk and the root flare which is where the trunk meets the soil. The root flare is at or near the ground line and is identifiable as a marked swelling of the tree’s trunk where roots begin to extend outward. Unless the soil is poorly drained, no more than a 2 to 4 inch layer of organic mulch should be used to prevent harm to the root system. When using finely textured or double-shredded mulch that allow less oxygen through the root zone, no more than 1 to 2 inches of mulch depth is required.
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