Primetime Newspapers
By Calvin R. Finch, PhD,
SAWS Water Resources Director, and Horticulturist
Week of June 5, 2006 
“Mulch is Important”  

            Mulch is an important part of landscaping and gardening in the San Antonio area.  Mulch prevents weeds, conserves water, keeps the soil cool, improves soil condition, can be decorative, and protects soil from erosion. 

            We have to be careful not to overstate the ability of a mulch layer to eliminate all weeds.  Depending on the depth of the mulch some weeds manage to grow.  For a weed seed to germinate, the seed must usually reach the soil.  Four inches of leaves or other mulch is more likely to keep seeds from reaching the soil than two inches is.  Some seeds can germinate within a wet layer of mulch and some perennials are capable of moving through and emerging from a mulch layer.  Overall, however, a layer of mulch will reduce the number of weeds and certainly make weeds that do emerge, easier to remove. 

            Water in the soil escapes two main ways.  A portion is drawn into the plant, used in its growth processes and escapes into the air as transpiration.  The transpiration stream from roots to leaf stomata and into the air is the main nutrient intake and transport mechanism of the plant.  Evaporation from the soil is water waste that can be reduced by mulch.  Shredded brush, pecan shells, bark, leaves, and even rock will insulate the soil from the air, and break the direct evaporation route.  For water conservation, use the mulch which has an appropriate texture and at a depth that matches the plants that are being mulched.  Small seedlings and seeded rows do best when a fine mulch is used at one or two inches deep.  A thin layer of compost (decomposed organic material) will insulate the soil, and not bury the seedlings.  Four to six inches of shredded brush over the root system of a newly planted tree will reduce watering needs and increase growth rate significantly.

            Many landscapes in the San Antonio area have very shallow soil or dense compacted soil.  This condition means, at best, that the soil reservoir that supports root growth is limited.  Without mulch the depth is reduced even more because bare soil heats up to120° F or more in the South Texas sunlight.  Roots do not function or even exist in some situations when the surface is this hot.  Mulch reduces the temperature by as much as 40° F with the result that roots can exist, and collect water and nutrients from more of the soil profile. 

            In compacted dense soils, a layer of mulch can improve texture and air movement by encouraging more root growth.  The decomposing mulch at the soil/mulch interface also encourages the activity of beneficial microorganisms that improve soil characteristics. 

            Sometimes we overlook the contribution that mulch can make to landscape aesthetics.  Visit the Water Saver Lane Exhibit at the San Antonio Botanical Garden to see some unusual mulch materials such as crushed grass and crushed toilets to see if their appearance appeals to you.  I find pecan shells and cocoa shells very attractive.  Even live oak leaves have appeal as a soil cover.  Various colors and shapes of rock can be used to provide interesting patterns while they function as mulch.  

            Mulch is often effective for erosion control.  The rock described in the previous paragraph is usually heavy enough to keep soil from flowing downstream in a natural drainage situation when the rains come.  Shredded brush also has enough overlap and intertwining to stay in place during heavy rainstorms.  Both mulches keep the soil where you have placed it.  Use shredded brush to cover large expenses of ground while you wait for plants to fill in the area.  Mulch used in that capacity is an eligible groundcover for the SAWS Water Saver Rebate.  The strategy allows individuals converting their landscape to a xeriscape to receive the rebate even if they cannot afford to plant all the trees, shrubs, and perennials until later.