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For The Garden

Cover Story

Author: Cathy Babcock
Issue: December, 2017, Page 112
When well-chosen, mulch is good for gardens

Compost and mulch are often referred to and used interchangeably. But the truth is that they are quite different and are useful for very separate purposes. So what is the difference between the two?

Compost is organic matter that has been “heated up” and broken down into a dark, crumbly material. This product is then incorporated into the ground and garden beds by turning it into the soil. When planting, compost can be mixed into the backfill and added to planting holes. It is nutrient-rich and works its magic best when it is in the soil where plants can absorb its benefits. Compost is considered a soil conditioner or amendment rather than a fertilizer. It improves soil structure by helping it retain moisture and nutrients, making these elements more readily available to your plants. It can be used as mulch, allowing the nutritional components to leach into the soil, but is usually a finer material than most mulch and will break down quickly.

Mulch, on the other hand, is a layer of material—organic or inorganic, as listed below—that is placed on top of the soil as a protective cover. Homeowners should be aware of the source of any mulch they use, if it is not commercially bagged. If acquiring mulch in bulk from a noncommercial source, such as a landscaper, be sure to ask what the product consists of and where it was procured. There are many applications for mulch. In the Valley, gardeners use it to reduce water loss through evaporation, thus retaining moisture in the soil. It also helps moderate soil temperature, acting as an insulator for soil and roots. The porous surface of mulch aids in reducing erosion, especially in a sloped landscape. It can also help control the weed population. Mulch does not have to be organic in nature, but if it is, it will add nutrients gradually over time as it breaks down.

What are the most common materials used for mulch?
Organic mulches that work well in the Valley include wood chips, sawdust, cottonseed hulls, hay, pine needles, grass clippings, leaves and straw. Some nitrogen depletion will occur with fresh wood mulches, so keep wood chips and sawdust out of garden beds and use only on pathways for this reason. As stated previously, know where your organic materials originate. Some wood products may come from trash wood, such as ground-up pallets that may contain creosote or other toxins. Do not lay wood products adjacent to the edge of the house as this is an invitation for termites to enter your home. Uncomposted organic material can also contain weed seeds, pet waste and chemicals.

Inorganic mulches can be stones, gravel or even rubber. They do not decay like organic mulch does and therefore do not need replacing on a regular basis. But they also do not offer any nourishment. Rubber mulches are not recommended as they will leach zinc and other pollutants into the soil.

There are several design considerations to take into account when choosing the type of mulch to use. Your garden setting will determine this. If your landscape is formal, wood chips or stones are a great way to create a uniform look throughout, giving visual unity to the landscape. Vegetable gardens and raised beds are often dressed with hay or straw. For a pathway or handicap access, consider crushed stone, upon which one can easily walk or ride. Many Valley homeowners have eliminated grassy lawns in favor of xeriscape landscaping, top-layering their properties with decomposed granite, which in itself is an inorganic mulch. To maintain a natural desert look in your yard, it would be incongruous to use wood chips or one of the other available organic mulch choices. Whichever mulch you choose, it should be spread to 3 inches deep and kept about 2 inches away from the stems of plants to prevent them from rotting and dying.

Mulches of various kinds can be purchased at any nursery or garden center. Many areas have a composting or mulching program for your yard waste; call the city in which you live to find out more. The City of Tempe offers its residents free bags of compost and mulch. Some tree and landscape services offer free wood chips; just check online.

Although not a landscape necessity, mulch can certainly help plants survive our searing summer heat and perhaps lower your water bill. Spread it around and spread the word.

Cathy Babcock is the director of horticulture for Boyce Thompson Arboretum, located in Superior.
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