Issue: May 4, 1998

What is mulch?

You recently wrote about the benefits of black plastic mulch versus clear plastic mulch. My question is about whether the mulch is tilled into the soil to capture the sun's infrared rays below the surface...or whether we simply lay the black plastic over the surface of the soil.

I always thought of mulch as something that is mixed into the soil.

The English language can sometimes be fun, or sometimes a real pain, because it changes so rapidly. I was using the term "mulch" in a traditional sense, as do most professional horticulturists. I have noticed that in recent times many people, especially home gardeners and the media, use the word "mulch" when traditionalists would say "compost" or "soil amendment".

I believe the confusion has developed because when we create a mulch from garden trimmings and debris, we encourage people to shred or grind it. This shredding or grinding has begun to be called "mulching" by many people, including some landscape professionals. We also encourage the shredding or grinding (mulching) of garden debris before adding it to a compost pile. Here is where the problem begins to develop. Then, once the compost has been formed by decomposition of the garden wastes, it is applied to the garden. It is usually tilled into the soil but may sometimes be applied to the surface as a "mulch". This is a further opportunity for "word confusion". The application of the word "mulching" to the grinding process and the use of compost as a mulch has allowed enough confusion to cause many people to call compost "mulch".

To a horticulturist, a mulch is a material applied to the surface of the soil to reduce the evaporation of water, to moderate soil temperatures, to reduce wind and water erosion, sometimes even to create an aesthetically pleasing background for the plant, and to limit weed growth. This term, mulch, is the same whether the mulching material is plastic, rocks, sand, or if it is shredded landscape debris. We use the term compost to describe the decomposed organic materials which are added to the soil as a soil mendment.

So, the plastic mulches I discussed are to be placed over the surface of the soil and not incorporated into the soil.

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!