Every landscape designer is asked the same question every year, “Should I use rock or wood mulch?” When faced with this question, it is always better to dig a little deeper without just blurting out an answer. Every individual situation is different, and people need the facts that will have them best equipped to make this decision for themselves.
The purposes of a ground cover like rock or wood mulch are weed suppression, water retention and aesthetic appeal. Depending on a person’s goals, sustainability and soil conditioning may also be factors that contribute to the decision. The best way to guide someone is to provide the facts, and see what the most important factors really are.
Rock ground covers, such as river rock or crushed granite, definitely have staying power. Rock is much more resilient to the elements, which plays into the sustainability. Wind, sun and rain are not going to speed up degradation or displace it like they do with wood mulch. Rock does not require topdressing every year and is easy to reuse. It can also be washed clean with a basic garden hose. Most rock products with a minimum diameter of 1/2″ or greater allow a lot of light and air through, so they require the use of a weed prevention barrier between the rock and the soil. This barrier is usually a poly-spun weed fabric or plastic sheathing. Without some kind of barrier, weeds can easily flourish through the rock. On the flip side, since rock requires a barrier, there is less airborne water passing through to the soil, whether naturally-falling or from irrigation spray. This makes it difficult for plant material to take advantage of all available water in dry conditions. Another drawback to rock is the heat factor. Rock gets much hotter than wood mulch and reflects sunlight, which can create an uncomfortable micro-climate for plant material. Rock, while easy to maintain, can be tough on plant material.
Wood mulch, such as shredded cedar or basic mulch, is extremely beneficial to soil and plant material. Wood mulch should have direct contact with the soil, because weed barrier will defeat most of the benefits. Wood mulch retains water, provides organic matter to the soil, harbors beneficial microorganisms, and reduces reflective heat. Even though a weed barrier is not used, with adequate coverage, wood mulch also suppresses weeds. It does a much better job of keeping light away from the soil. Wood does have a few drawbacks, though. It is easily displaced by both heavy wind and excessive surface water, creating a potential maintenance nightmare in exposed areas. When it is displaced, all of the weed barrier qualities are also void, and the weeds in the exposed soil can get out of control. In areas with critter issues, it can be moved around much easier than rock. Wood mulch needs to be topdressed with a fresh layer every year to maintain it’s look, feel, and integrity. The total thickness of the layer should always be around 3 inches deep. Wood mulch, while being beneficial to the horticultural environment, can require more maintenance input.
Environmental factors can also create different psychological effects with ground covers. To the human mind, rock can feel formidable, strong and secure, but also hard, dry, and uncomfortable. It has no real “life feel” to it. Wood mulch feels soft, organic and nurturing, and also has that fresh cut wood smell, especially after a cleansing rain. However, after a year or so, it starts to look gray, dingy and dead, which is another reason it needs to be topdressed every year. People are amazed at how prior life experiences can affect their happiness with their landscape ground cover.
Good landscape designers will look at the exposure, placement, and overall purpose of the landscape bed area when deciding what type of ground cover to use. Sometimes, it can even be dictated by the specifications of a governing body, such as a municipality or homeowners’ association. However, when deciding for themselves, property owners need to examine all of the facts and choose the groundcover that best fits the purpose and feel that they are looking for in each specific landscape bed.