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Murder by Mulch

overmulched tree.jpg
MURDER BY MULCH:  how much is too much?

Amy Dismukes, UT/TSU Williamson County Extension

​Mulching is an incredibly beneficial landscape practice, if done correctly.
With the season upon us, it’s important to remember that mulch can also be detrimental, if too much is utilized.


Applying a 2 to 4-inch layer of organic mulch can mimic the natural forest environment, where tree roots are provided a rich, oxygenated soil, full of nutrients and beneficial microorganisms. The forest floor is basically nature’s way of composting, replenishing and recycling nutrients without human intervention. Good soils make for good roots, resulting in healthy trees.


Organic mulches include wood chips, pine needles, hardwood and softwood bark, cocoa hulls, leaves, compost mixes and a variety of other plant products. Decomposition rate is dependent on the product used. Those that decompose faster must be replenished more often, however, the benefit of soil quality improvement generally allows gardeners to turn a blind eye on the additional maintenance.


Additions to mulch should be made only to maintain proper depth. Fluffing the older layer of mulch, before adding more, can prevent the formation of a hard surface that may deflect water rather than retain it.


Inorganic mulches are also available and include stone, lava rock, pulverized rubber, geotextile fabrics and others. Unlike the organic mulches, they do not decompose and therefore don’t require frequent replenishment. On the flip side, inorganic mulches do not improve the soil structure or provide desired nutrients that organic mulches do.


Perennial groundcovers are often used as a mulch alternative. Some gardeners even utilize colorful annual flowering plants in circular beds around the base of trees as a substitute for mulch.


Proper mulching has a plethora of benefits. It reduces soil moisture loss and provides insulation, protecting roots from extreme temperatures. Mulch aides in weed control by providing a barrier between the seed and the sun.


It can also deter fungal pathogens. Many fungal diseases that affect plants (leaf spots, etc.) will over-winter or hibernate on fallen plant leaves and/or twigs, when conditions become unfavorable. As soon as the environment stabilizes and the pathogen becomes happy, it will pop right back up to re-infect. Rule of thumb: if the issue starts on the lower foliage or branches, you’re probably looking at a fungal disease.


Mulch can improve soil tilth (structure), fertility and drainage via decomposition. Like compost, mulches (wood chips, pine straw, hardwood, etc.) are organic matter and will break down over time, recycling nutrients back into the soil.


More importantly, mulch can also reduce the potential of damage from lawn mowers or weed whackers. Wounds allow for the entry of insects and disease, both of which have the potential to cause serious damage.


However, against the normal train of thought (and current landscape trend), more is not always better. As beneficial as mulch is, too much can be harmful. Especially for our beautiful trees. Although the generally recommended mulching depth is 2 to 4 inches, many landscapes are falling victim to a plague of what we in the industry term “volcano mulching” or excessive piles of mulch materials applied around the base of trees.


Excessive mulch piled up against the base of a tree keeps moisture in direct contact with the bark, which suffocates the cells of the phloem, the layer of living tissue that transfers food up and down the plant. When this supply of food is limited, the roots die back. This leads to less water uptake, and the tree goes into general decline, leaf drop and potentially premature death.


Secondary problems, such as wood boring insects and fungi, move into plants weakened by improper mulching. Voles, also known as field mice, tend to migrate to deep mulch rings and chew at the bark and root system of the tree.


If trees have been over-mulched too heavily, remove the excess mulch using a shovel or trowel. Be careful not to injure the trunk. New mulch can then be applied properly … and remember, more is not always better!!