We Can All Agree on Mulch — Or Can We?

garden in Claremont, CA. with decomposed granite mulch and river rock

From Wild Suburbia

Most people do not realize that gardeners are a contentious bunch. They discuss, banter, argue, and almost come to blows about which plants, amendments, and gardening practices are best. There is one thing, though, that most agree on, and that is the wonders of mulch.

Mulch is a top-dress for soil. It can be organic, such as woodchips, bark, or partially decomposed compost. Inorganic mulch can consist of rocks, pebbles, or decomposed granite. Mulch is even made from shredded tires.

A few of the many benefits attributed to mulch are moderation of soil temperature, water conservation, weed and erosion control, and improved soil structure. All of this, while providing the garden with a look that is neat and cared for.

Types of Mulch
When selecting mulch for native plant gardens, if possible try to observe your plants in their natural settings, or check with a knowledgeable landscaper or person at a local nursery. Most undisturbed sites have some kind of soil covering. Woodland soils are usually covered by a deep layer of organic material. Chaparral and scrubland have little organics on the soil, though some do collect, along with pebbles and inorganic debris. Sandy desert soils are covered with a thin green layer called a cryptogamic crust. This crust—composed of cyanobacteria, algae, moss, and lichens—is critically important for the maintenance of a healthy desert ecosystem.

Using organic mulch in gardens that feature desert, chaparral, or scrub plants can produce some undesirable effects. If the mulch remains too moist, these plants are more likely to rot. Furthermore, in time organic mulch enriches soil. Dry plants are adapted to lean soil. In enriched soil they may respond with lush growth that dehybrates during hot weather , resulting in extreme stress or even death.
On the other hand, plants adapted to wetter, more verdant regions appreciate a layer of organic material. Inorganic mulch can get too hot in summer, too cold in winter, and does not hold water nearly as well as organic mulch.

With regard to using shredded tires as mulch, Linda Chalker-Scott of Washington State University concludes in her review of scientific studies that this is an inappropriate material for the following reasons: 1) it is not as effective in controlling weeds; 2) it is highly flammable and difficult to extinguish once it begins burning; and 3) it contains a number of metal and organic contaminants with known environmental and/or human health effects.

Caution
In addition to matching the mulch to the plants, here are a few other advisories. Always start with clean mulch. Keep mulch away from trunks or stems of plants. This area, called the crown, is susceptible to pathogenic organisms and should be kept clean and dry. If using a thick layer of organic mulch, make sure you water enough to soak the soil below the mulch, and then give the soil a chance to dry out. Avoid mulch that may contain weed seed, and pesticides or other toxins. A thin layer of organic mulch can encourage weed growth, so be sure to keep the organic mulch three to six inches thick.

And Don’t Forget the Bees
And finally, don’t forget the bees! Mulch in gardens and parks reduces habitat for native solitary bees, since many of these insects nest in bare soil. With the decline of European honey bees, and studies indicating that solitary bees make a significant contribution to the pollination of both crops and native plants, it may be prudent to leave some soil undisturbed and unmulched.

solitary bee (Halictus tripartitus) in early morning (photo by Hartmut Wisch)

pollen-laden female of same species (photo by Hartmut Wisch)

female bee (Diadasia sp.) excavating burrow (photo by Hartmut Wisch)