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New Mexico State University

Issue: May 9

Which is better? Rock mulch or bark mulch?

Q. What mulch is best in New Mexico? I have had some people say wood chips are best, but I notice a lot of rocks in landscapes.

Tony G.

Rio Rancho

A. Mulch is an important part of wise landscaping. Both organic and inorganic mulches can help conserve water, encourage rain water to permeate the soil by reducing compaction, inhibit (not prevent) weed growth, and reduce soil erosion by wind and water when used properly. However, each has its own benefits and liabilities.

The best mulch depends on the way it is used in the landscape. Perhaps the first consideration should be which mulch will stay in place. Straw and dried grass clippings are good mulch materials, but they blow away too easily in most situation. Wood chips are less likely to blow away, but if they are in an area that will have a flow of water during a heavy rainstorm (under downspouts and canali), they will float away. Rocks are best in this location. If the slope is great, larger rocks or even cobbles are worth considering.

It is also important to consider the plants in the landscape. Rocks will accumulate more heat than organic mulches and may harm some plants. Those plants may require organic mulch (straw, wood chip, bark, etc.). Most plants will benefit from organic mulches around their base and inorganic mulches (rocks, gravel, cobbles, etc.) in areas farther from the plants. However, some plants native to the Southwest and similar regions may require a highly porous, highly aerated soil and may suffer from carbon dioxide released into the soil pore spaces as organic mulches decompose. Such plants require inorganic mulches.

The final consideration is your preference regarding how each type of mulch creates its own particular appearance in the landscape. You should also consider your preference of plants and how these plants appear with specific mulches. Because inorganic mulches tend to increase heat buildup in the landscape's environment, consider plants that will shade them as much as possible to reduce the heating. Different colors and sizes of mulch materials will provide appearances in the landscape.

For something novel in your landscape you can consider rubber mulch (often made from discarded tires from which the steel has been removed). These are available in several colors to enhance the landscape. They can provide a safe, clean play surface for children and pets). However, rubber mulch can become hot in the mid-day heat, too hot for children to play on. Rubber mulch has many of the advantages of both organic and inorganic mulches, some liabilities of its own, and the benefit that it uses what would otherwise be a waste product.

For more gardening information, visit the NMSU Extension publications World Wide Web site at

Send your gardening questions to:

Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd. SW
Los Lunas, NM 87031

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.