A mulch, in garden terms, is a layer of rock or organic matter placed over the soil in and around plants. A mulch layer benefits plants in several ways.
Mulch conserves moisture and keeps the soil cooler during the hot spring and summer. In addition, mulches prevent erosion and help discourage weed growth. If organic mulch materials are used, soil quality is improved over time.
The most common mulch used in desert landscapes is decomposed (weathered) granite, also referred to as "DG." This type of mulch has a mixture of weathered rock from very fine soil-like particles to pieces up to three-eighths of an inch in diameter. Crushed granite is also used and is available in a variety of colors and sizes. The color variation is attributed to the location where the granite rock is mined.
Decomposed and crushed granite are inorganic forms of mulch and, as such, are not capable of improving soil quality. However, they do help insulate the soil, holding in moisture and keeping the soil cooler by several degrees as compared with bare earth.
For best results, the layer of granite mulch, either decomposed or crushed, should be from 11/2 to 2 inches thick. The rock mulch will gradually sink into the soil with heavy monsoon rains and foot traffic, so it will need to be replenished from time to time. To maintain its effectiveness as a mulch, a thin layer of new rock should be added in areas where the soil begins showing through.
Even though rock mulches are the norm for desert landscapes, organic mulches may also be used. In fact, the natural mulch out in the desert is a combination of gravel, rock and organic matter in the form of fallen leaves, twigs, stems, flowers and decaying plants.
Wood chips and ground wood mulch are the best organic materials for landscape use. They do a better job of holding in soil moisture and holding down soil temperatures (by as much as 10 degrees) than rock mulches. In addition, as they decompose, beneficial organic matter is added to the soil. Our Xeriscape Demonstration Garden at the Extension Center is mulched entirely with ground wood, and the plants, especially our desert perennials, really thrive as a result. For best results, wood mulches should be applied in a layer 3 inches thick.
Garden beds of perennial and annual flowers and vegetables require the use of organic mulch. It's best to use organic matter that will readily decompose and can be mixed into the soil as seasonal flowers and vegetables are removed and replaced.
Straw is an excellent mulch material for garden beds and can be readily obtained at area feed stores. Even if you don't need a full bale of straw for mulching, you can take along a grocery or trash bag, and most stores will let you fill the bags with loose straw for a small charge.Β Straw is applied in a layer 3 to 4 inches thick. Other good materials for garden mulching are bagged or bulk compost, well-composted manure, pine needles, mesquite straw (stems and leaves) and sheets of newspaper.
Keep in mind that organic mulch materials have to be replenished as they decompose. Wood mulches, such as ground cedar, will last two or three years, but fine materials such as compost break down after just a few months.
β John P. Begeman is the urban-horticulture agent for the University of Arizona-Pima County Extension. If you have questions, call 626-5161 to reach a master gardener.
Β» project
"Making Your Own Patio Water Garden" will be the topic for this week's garden demonstration. It will be presented Wednesday at 9 a.m. at the University of Arizona-Pima County Extension Center, 4210 N. Campbell Ave.; and at 1 p.m. at the Wilmot Library, 530 N. Wilmot Road. Also on Friday at 1 p.m. at the Oro Valley Library, 1305 W. Naranja Drive.
