Issue: November 22, 1999

Mulches -- plant responses

What is the best type of mulch to use as a top layer over soil?

This week the answer continues from last week when we discussed the characteristics of various mulches. This week we will focus on the characteristics of plants as they relate to selecting the appropriate mulch. This is somewhat less complicated, but also the characteristics of specific plants are sometimes less well known.

Many plants grown in New Mexico landscapes are not native to New Mexico. The majority of these are from regions which are cooler, moister, and have less alkaline soils. The appropriate mulch for these plants should help alter the environment and soil to simulate that from which the plants are found naturally. Some plants will adapt to a wide range of conditions - these are the easiest to grow under our conditions. Other plants have a very narrow range of environmental adaptability - these plants are much more difficult to grow under New Mexico environmental conditions. Proper choice of mulch material is especially important with those plants with less environmental adaptability.

Organic mulches maintain a lower temperature around plants and are best for plants from cooler locations, whether from cool locations outside New Mexico or high elevations within our state. These organic mulches will decompose, and in the process neutralize some of the alkalinity of the soil. The carbon dioxide from the decomposition of these mulches dissolves in the soil water, lowering the pH of this soil water.

If it is desirable to use inorganic mulches around plants native to cooler climates, it is important to provide shade. Inorganic mulch may be used around plants requiring cooler conditions if the plants are in the shade of buildings or other structures. It may also be possible to plant other, larger plants to provide the necessary shade and cooling.

Plants native to New Mexico soils, especially those from sandy and gravelly soils such as are found in arroyos, are often dependent on the porosity of the soil to provide high levels of aeration and high oxygen levels around the roots. The carbon dioxide from decomposing organic mulch is often harmful to these plants. Such plants benefit from an inorganic mulch of cobbles, gravel, or crusher fines, a byproduct of the crushing of rocks, to make gravel. Plants native to arroyo environments are often adapted to the higher temperatures present above the inorganic mulch.

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!