Weekly Express-News Article
By Calvin R. Finch, PhD, SAWS
Water Resources Director, and HorticulturistSaturday,
June 3, 2006 
“Conserve Soil Moisture with Mulch”  

            This last week we had some great rain.  One of the best ways to conserve the moisture that reaches the soil is to use mulch to insulate the surface from the evaporative pressures that result when the hot sun and dry air make contact with the soil.  The mulch provides and insulating layer that reduces evaporation significantly.

            In addition to breaking the direct contact between the moist soil and dry air, the insulating mulch layer reduces soil heat.  Soil under direct rays of the sun can heat up to well over 100° F to depths of two to four inches.  Many plant roots cannot function at those high temperatures so you can have a “dead zone” due to soil temperature.  The problem is made more severe because in many South Texas neighborhoods, we do not have more than four to six inches of soil.  The reservoir area for active root growth and resultant water and nutrient collection can be very limited.  Even if you have more soil than four to six inches, it is often a heavy clay that is too compact and devoid of oxygen for root growth anyway.  The idea of deep root systems and tap roots in our soils is a myth.  Research has revealed time and time again that 80% of root activity is in the upper six inches of soil.  Keep the sector cool enough for roots with mulch.

            Mulch does not eliminate weeds, but it reduces the number of weeds, and makes those that appear easier to pull. 

            Decomposing organic mulches contribute nutrients and provide an environment that encourages micro-organisms to flourish.  Why waste money adding micro-organism stimulators to your soil when the key to high beneficial population is plant growth and organic material, such as that provided by organic mulches.

Use two inches of mulch for small plants and four to six inches for trees or shrubs.

Leaves – Leaves are my favorite mulch.  We have two leaf drop seasons to supply foliage for mulch.  The live oaks drop leaves in the spring and our deciduous trees (pecan, red oak, and others) drop leaves in the autumn.  All leaves make good mulch.  Collect your own with a rake or a bagging lawnmower and use them two inches deep for the paths in the vegetable garden or four inches deep in the shrub border.  If you have a pick-up truck, collect the bagged leaves left out for the refuse collection by your neighbors.  They are too valuable for the landfill.

Shredded Brush – Once chopped, shredded brush is available at the City of San Antonio brush site at 1800 Bitters Road. Twice chopped is easier to use (less big pieces).  The City charges three cents ($.3¢) a pound for it.  Horticultural supply firms like Keller Material, Fertile Garden Supply, and Garden Ville offer the material for about $25/cubic yard.  Add about $7/yard for delivery in five yard units.  Shredded brush is especially good for mulching on slopes or other locations where mulch might wash away.

Pecan Shells – Pecan shells used to be free if you picked them up at a shelling facility, now, they cost about the same as shredded brush.  They are attractive and easy to move.  New shells attract deer, fire ants, and other critters seeking remnants of the nuts in the shells.

Rock – Inorganic mulches work well in some situations.  Decorative rock is available in many colors, sizes, and textures. It does not decompose like organic mulches and lends itself to a desert look and extreme low maintenance.

Fabrics – Weed barriers are used under both organic and inorganic mulches.  They reduce weed growth while allowing air and water to travel through the barrier.  They are almost always covered with a decorative mulch like rock or pine bark.  Fabric weed barriers work well for some situations, but are not weed-proof.  They are unsuitable if you move plants around or work in the bed to any degree. 

Compost – Compost is decomposed organic material.  It does not matter what the source material is - leaves, manure, grass clippings, or sawdust, the final product of the decomposition is the same; a clean smelling, loose brown material that is an ideal growing medium.  It works as mulch, and also a great soil additive.  Two or three inches of compost incorporated into the soil can increase drainage and water holding capability.  Compost is so valuable as a soil improvement material it is probably best used for that purpose.  As a mulch it sometimes floats away, but can be covered with a heavier coarse material like shredded brush.