Mulching provides many benefits for trees and shrubs, including moderating soil temperatures, reducing soil moisture loss and reducing soil compaction. Mulch also provides nutrients, improves soil structure and reduces winter desiccation. These benefits result in more root growth and healthier plants.

The most important benefit of a properly installed mulch ring is the protection zone that it creates around the base of the plants. Protection from the bad-tempered string trimmer or a misguided lawn mower is a greatly appreciated treat for a tree. I couldn’t begin to count the number of trees that people have killed mowing the lawn. This damage can be fatal, especially to trees that are already stressed or of a sensitive variety.

When applying mulch, the following guidelines should be observed:

  • The best mulch materials to use are composted leaves, wood chips, bark nuggets or pine needles. Avoid plastic, stone, sawdust and grass clippings. While rocks can be low maintenance, they give nothing back to the trees and shrubs.
  • Mulch should be applied from the dripline to near the trunk. If this is not practical, minimum mulch circles should be three feet for small trees, eight feet for medium trees and twelve feet for large trees.
  • When applying mulch to existing trees, it is best to mow the grass near the ground and cover it with mulch. Removing the sod may cause significant root damage to shallow rooted plants, though some grass can grow through the mulch.
  • Avoid using plastic barriers that are designed to prevent weeds. Barriers trap moisture, suffocate roots and prevent organic matter from the decomposing mulch from being incorporated into the soil.
  • The mulch should be applied two to four inches thick.
  • Do not remove old mulch to replace with new mulch. The decomposition of the mulch is improving your soil. You can add some mulch annually to pretty up the mulch beds but maintain no more than a two to four inch depth.
  • The mulch should not be placed against the trunk or above the root collar of the plant.

The root collar is the area where the trunk and roots come together. It is the location on a tree trunk where the roots begin to grow laterally into the soil, also known as the root flare. On all woody plants, it is critical to keep this area exposed. Exposed meaning not covered with soil or mulch. There are many reasons why the root collar must be visible. The fact that the plant may die is the easiest to remember.

Trunk tissue and root tissue have different characteristics. For the most part, root tissue can resist constant moisture, trunk tissue cannot. The result of constant moisture on the trunk (of species not tolerant to swampy environments) leads to decay and possibly death of the tree.

If soil or mulch is covering the trunk tissues, they will suffocate and die. Eventually pathogens will invade the tissues of the stressed plant and potentially kill the plant. Phytophthora, and Armillaria are common root rots that can attack root collars and roots with declining phloem.

Mulch provides many benefits if applied properly. If mulching is applied improperly the consequences can be dire.

Cody Kouneski is an Arborist Representative for Bartlett Tree Experts and is a ISA Certified Arborist. Email questions about your own trees and shrubs to ckouneski@bartlett.com or call 570-922-0113

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