Water-Efficient Landscaping in the Intermountain West: A Professional and Do-It-Yourself Guide

Principle 5: Use of Mulch

71 Principle 5 Use of Mulch Heidi A. Kratsch and Margaret Shao Why Mulch? Mulching around plants in a water-efficient landscape helps keep valuable water in the soil around plant roots and discourages the germination and growth of weed seeds. This not only minimizes irrigation requirements but results in a landscape with low or less frequent maintenance needs and reduces pesticide use. Hoeing or spading around plants to remove weeds can damage plant roots, disrupt soil structure, and discourage the presence of beneficial soil organisms . Mulching won’t completely eliminate the need for weed control, but it can decrease the amount of time and labor used for this task. Mulching also moderates soil temperatures and prevents soil compaction. The presence of mulch promotes root development near the soil surface, increasing water, nutrient, and oxygen availability to the plant. Mulching around landscape trees can reduce competition between tree and turfgrass roots for soil water and nutrients. Finally, mulch can be an attractive design component in a water-efficient landscape. Types of Mulch Organic mulches include materials such as wood or bark chips, shredded bark, nut shells, pine needles, sphagnum peat, and grass clippings or other discarded disease- and weed-free plant parts. Certain materials may be more available than others on a regional basis, and their availability and price may dictate which 72 Water-Efficient Landscaping in the Intermountain West types are used. Organic mulches have the potential to enhance soil structure, increase soil fertility, and increase soil organic matter. Using organic mulch is a great way to recycle yard waste such as pruned woody plant materials and fallen leaves and needles. To ensure adequate aeration and water infiltration and to slow decomposition, make sure mulch particles are larger than the soil particles below (usually larger than one-half inch in diameter). However, some organic materials such as leaves or lawn clippings can mat together and restrict air and water movement into the soil when left whole; they should be chipped or mulched into one- to three-inch pieces before applying as mulch around plant roots. It is important to be sure recycled plant materials are free from weed seeds, disease-causing organisms , and pesticide and herbicide residues. One way to prevent harm to plants by mulch is to use disease-free plant parts that have not been treated with pesticides. Another way is to compost mulch before use. Composting results in controlled decomposition of organic materials through the activity of microorganisms (bacteria and fungi). When done properly, composting generates enough heat to kill most weed seeds and disease organisms, and will break down some pesticides. In most cases, use of organic mulch increases the nutrient fertility of the soil below. This decreases the need for application of fertilizer to mulched plants. However, the use of fresh wood or bark as mulch may lead to nitrogen deficiency because microorganisms that decompose these materials use up the nitrogen that would have been available to the plant. This nitrogen loss can be avoided by using composted mulch, or by adding nitrogen at a rate of one-half to one pound per 1,000 square feet prior to applying mulch material. Over time, organic mulches break down and will need to be replenished. This can be accomplished by adding more mulch over the top of the decomposed mulch material. Always “rough up” the old decomposing mulch before applying new mulch materials to prevent a water-impervious layer from forming between old and new mulch layers. Although the decomposition of old mulch adds to the soil organic matter, it can lead to nitrogen deficiency. Watch plants for telltale signs of this, such as leaf yellowing that starts at the bottom of the plant and is first apparent in the older leaves on the stem. Replace lost nitrogen when necessary using the previously described application rate. Inorganic mulches consist of nonplant materials and may include a variety of different kinds of rock or gravel. Different sizes and colors of inorganic Figure 5-1. Wood chips should be composted and free of weed seeds and pesticide residue. Photo: Heidi Kratsch Use of Mulch   73 mulches are available. The decision about which to use depends upon the design of the landscape, the types of plants used, and availability. Examples include gravel or crushed stone, cobblestones, river rock, and lava rock. The texture of inorganic mulch should complement the scale and type of landscape. For example, cobblestones work great when used judiciously among trees and hedges in an...