Home & Garden

August 1, 2013

Local gardening: Dangers of mulch volcanoes

Amanda McNulty: For mulch to promote plant health, it needs to be used correctly.

Remember those terrifying movies when the volcano that had quietly simmered for years on a tropical paradise began to spew and sputter? A beautiful young maiden was selected to appease the angry gods and was 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.

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 had indeed been mulched. In volcano mulching, bark, pine straw, or other material is piled up in a cone-shaped structure on top of the newly established plant, 8 to 12 inches high and extending 3 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 , 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. Also adding to the seemingly endless list of problems volcano mulching can cause is the fact that the excessive depth of the mulching material may actually cause it to compost; generating 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 material. 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 trees may become colonized by certain fungi that actually repel water and create a hydrophobic area, completely negating the moisture-retentive benefits of a correctly applied mulch, and leaving your poor tree high and dry even though it’s under an irrigation system.

Take time when you’re mulching to keep the material you’ve chosen at least 5 inches away from the trunk of trees and 2 to 3 inches from stems of woody shrubs.

After settling, mulch should be between 2 and 3 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 lost 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 8 inches of pine straw.

With the excessive rains of this summer, over-mulched plants are in increased danger of drowning or rotting. Take advantage of the occasional dry days we get to check the plants in your yard, pulling mulch away from trunks and stems and removing excess mulch that may have washed or been applied by a heavy-handed worker. Don’t let your beloved trees be sacrificed to a false god of ignorance. For mulch to promote plant health, it needs to be used correctly. See Clemson’s HGIC factsheet 1604 Mulch for more information.


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