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Mulches can save water, but too much is not good


Photo submitted by NEIL SPERRY

Pine bark mulch looks natural, and helps the ground to retain moisture as well as preventing weeds.

You can get too much of a good thing, and mulches are a great example. Applied too generously, they can actually impede the flow of water and nutrients into the soil. Spread in appropriate amounts, they can be real life-savers in a summer of intense water restrictions.

What on earth is mulch?

That’s exactly what it is — mulches are materials we use to cover bare soil. They can be organic (bark mulch, compost, dried grass clippings, shredded tree leaves, etc.) or inorganic (gravel, ground rubber tires, glass, roll-type fabrics, polyethylene, etc.). Organic types decay over time, returning nutrients to the soil. Inorganic types don’t decay, so they’re usually more permanent. Organic mulches look more natural, and they’re not picked up by lawn mower blades and hurled across the yard.

Why do we mulch?

Mulches reduce splashing, so they protect plants against soil-borne diseases that are spread during irrigation and rainfall. They keep soil from splattering the sides of our houses. People with iron-rich red soils are all too familiar with the stains that splashing can cause.

Mulches moderate the rates in which soil temperatures change in the winter. It’s the freeze-thaw cycles that do damage to plants. We learned as children that if we are at risk for frostbite, we should hold our hands or feet under cool — not warm — water for the very same reason.

However, the main reason most Texans use mulches is to conserve water. Mulches reduce soil-to-air contact. Just as wrapping loaves of bread keeps them from drying out, covering bare ground helps it to retain moisture. And as an added bonus, mulches make it much more difficult for weed seeds to sprout. Weeds are water hogs, so anything that retards their development will also save water.

Can mulches be bad?

It’s not that mulches themselves are bad. It’s how we use them. As is our custom, we seem to do things in excess. All you really need to add for mulches to perform the magic we just finished outlining is 1 to 1-1/2 inches. I see people pouring out layers 3 and 4 inches deep, thinking, I guess, that they’re providing much more of a great thing.

The same thing happens around young trees. Arborists refer to what they see as “mulch volcanoes.” They’re mounds 8 to 12 inches deep piled up around the trees’ trunks. Every year, more mulch is added.

So the problem in all that is water and nutrients never make it to the root zone. All that organic mulch absorbs water about as quickly as you can apply it. The mulch gets soaked thoroughly, but the ground is often still dry.

It does need to be noted that some tree-planting crews plant their trees “high” (above the surrounding grade), saying that it will ensure good drainage and aeration in wet times. They go so far as to pile the soil they remove from the holes up around the trunks. Then they conceal it all with the mulch. Truth is, this procedure is much better suited to southeast Texas, where water tables are often near the soil surface than it is to our area.

Join Neil Sperry Sunday mornings 8 until 10 on WBAP 820AM. He is author of Neil Sperry’s Lone Star Gardening and publisher of Neil Sperry’s GARDENS Magazine. Learn more at, and join Neil on Facebook.

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