Issue: December 7, 2002

Bark mulch

I would like to find out what kind of trees are the best to produce wood chips/mulch for gardening purposes. Also, can mulch act as a protection layer for soil so the soil won't be washed out when watering the plants? I really need your help for the purpose of landscaping/gardening. - Eean L.

Almost any kind of wood chips can be used as an organic mulch. There is a difference between bark and wood chips, but bark from most trees is also useful. In the Eastern part of the U.S., there is concern that bark, especially pine bark, increases the acidity of the soil. Here in the Southwest, that is not a concern. In most cases any acidification, though mostly beneficial, will be temporary.

Another concern is the presence of allelopathic compounds (chemicals produced by one plant to inhibit the growth of other plants). Of primary concern regarding allelopathy is the bark from black walnut trees. The bark and husks of walnuts contains a chemical, juglone, that inhibits the growth of plants in the Solanaceae (tomato/potato family). Research has shown no significant effect of the wood, but the bark may be a problem if placed around plants in the tomato family.

The benefit of soil protection you mentioned is only one of the benefits of mulch. On steep slopes and in rapidly flowing water, wood chips may wash away. In those locations a heavier mulch, perhaps rocks, would be best. In most other locations, wood chips are an excellent soil protector. Mulch prevents water and wind erosion and stabilizes soil temperatures (warmer in winter, cooler in summer under mulch). It also reduces evaporation of moisture from the soil, reduces weed competition with landscape plants, and prevents damage to trees and shrubs caused by lawn mowers and string trimmers. Mulch creates a better environment for many beneficial soil organisms. As it decomposes, it improves the soil. It also reduces soil compaction, increasing infiltration of water and air into the soil. This results in healthier tree and shrub roots and healthier trees and shrubs. These benefits also help herbaceous perennials.

Wood chips and bark are not the only beneficial organic mulches. Old straw or hay, dried grass clippings, pecan shells, and many other organic materials are useful as organic mulches. Inorganic materials such as rock, paving and flagstone, and other inorganic materials are also useful. Each material has its limitations and its strengths. For example, pecan hulls are a common New Mexico waste material useful in the landscape, so it is inexpensive. However, the pecan shells usually contain some residual pecan nutmeats which attract rodents and insects (roaches, etc.), a potentially negative characteristic that must be considered. Rocks don't wash or blow away as easily as organic mulch, but they become hotter and may injure some plants. Choose mulch materials with their strengths and weakness in mind, and choose appropriate sites in which to use them.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!