Volcano Mulch

Amanda McNulty,
Clemson Extension Horticulture Agent

Remember those terrifying movies when the volcano that had quietly simmered for years on a tropical paradise began to spew and sputter? To appease the angry gods, a beautiful maiden was selected and thrown into the fiery cauldron below.  At least she had a quick death. Trees subjected to volcano mulching may spend years in decline until their life finally ceases.

Volcano Mulch
Volcano Mulch
Amanda McNulty©2014, Clemson University

Maybe this damaging practice started so uneducated or lazy site managers could ride by and from the comfort of an air conditioned truck see that newly installed trees and shrubs had indeed been mulched. In volcano mulching bark, pine straw, or other material is piled in a cone-shaped structure around the newly established plant, eight to twelve inches high on the trunk and extending out three feet in diameter.

A tree’s trunk is covered with bark tissue that shouldn’t be subjected to constant moisture, unlike the root system which needs to be surrounded by a moist (not sopping wet) medium.   When the trunk of a tree is buried under mulch, it becomes soft, mushy, and prone to fungal diseases and rot. The all-important root-flare, the lower portion of the trunk which widens as it transitions into the root zone, (analogous to my waist line in the old days when it flared attractively into hips), should be above ground and free from any contact with mulch.

As part of preparing for winter dormancy, the tissues on the exterior of the trunk harden off. Mulch piled up high on the trunk acts like a down vest, preventing those cells from responding to fall’s cooler temperatures and leaving the plant unprepared for freezes. In addition to the seemingly endless list of problems volcano mulching can cause is the fact that the excessive depth of the mulch may actually cause it this material to compost and generate heat at tissue-destructive levels.

Since you shouldn’t amend the planting hole but place your new tree in the native soil, the roots of a newly planted tree covered by improperly dense and deep mulch may actually grow upward into the seemingly more nurturing mulch.  But when drought or freezes occur, those roots don’t have the insulating value of the soil to protect them.  Since fungal organisms are among the primary decomposers of wood, thick layers of mulch from ground up trees may become colonized by certain fungi that actually repel water and create a hydrophobic area. This may completely negate the moisture-retentive benefits of correctly applied mulch, and leave your poor tree high and dry even though you think it’s getting water.

Take time when you’re mulching to keep the material you’ve chosen at least five inches away from the trunk of tree and two to three inches from stems of woody shrubs. After settling, mulch should be between two and three inches deep.  Extremely fine-textured mulches that may be labeled “double ground” tend to pack down and should be applied more lightly than coarse textured materials that stay fluffed up. To get the most value from mulch, apply it all the way to the drip line or cover the entire planting bed.

Mulches from once living sources decompose over time and add beneficial organic matter to our soils. Pine straw, bark and other materials lose their attractive color over time. Although gardeners want a fresh look year round, and Clemson Extension agents tout the value of replacing organic matter to natural processes, don’t fall prey to thinking that every fall and spring you should add another couple of inches of mulch to your beds.  Evaluate the mulch levels before you blithely add more. We killed several trees in a specialty garden in Sumter by using every single bale of straw we had delivered; the delivery man had long since gone and no one had a truck. When two Forest Pansy redbuds began declining, we discovered the root flare of each one was buried under eight inches of pine straw.

In times of heavy rainfall, over-mulched plants are in increased danger of drowning or rotting. Take advantage of a dry day to check the plants in your yard. Pull mulch away from trunks and stems and remove excessive mulch that may have washed or been applied by a heavy-handed worker. Don’t let your beloved trees be sacrificed to a false idol of ignorance.  For mulch to promote plant health, it needs to be used correctly. See Clemson’s HGIC  1604, Mulch for more information.  

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