by Lydia Holley May 25, 2020
I intended to place a thick layer of tree-derived mulch in my garden this year, but with the pandemic, I have not gotten out to purchase mulch. Without a layer of mulch to prevent the weed seeds from touching bare earth, the weeds are winning the war. With the state slowly opening, mulch is on the top of my nonessential purchase list.
A one-inch thick layer of mulch will make a difference, but studies have shown that organic mulches are most effective at a two to three-inch depth. Mulches keep your soil cooler and help retain moisture. Some worry about mulches binding the nitrogen available to ornamental plantings, but that has not been observed in studies. Some also believe the soil pH can be changed through different mulches. However, one study applied eighteen inches of mulch, waited two years, and tested the soil. There was no change in the pH. So, do not worry about using a pine mulch. It will not acidify your soil.
Mulches applied deeper than four inches, however, can be detrimental. A mulch that is laid too thickly can prevent roots from taking hold. Studies have shown that five inches of mulch may keep plants from growing, may also reduce their root growth, and can introduce disease. If very thick mulch is laid next to woody stems, it may suffocate the plant. Some perennials such as peonies and irises may not bloom under a deep layer of mulch.
What is good to remember is that three inches of mulch does not have to be applied every year. Measure the depth of your mulch and top it off to reach a total depth of three inches. The annual rate of decomposition will be different in varying soils. Sometimes all you need to do is rake the top, fluffing the mulch to get that just-laid look without actually adding more.
Henderson County Master Gardener Association provides multiple resources at its website: https://txmg.org/hendersonmg/ or direct questions by calling the county extension office at 903-675-6130.
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