Mulch – Best Management Practices

When done correctly, top dressing tree rings and planting beds with mulch is one of the highest benefiting practices homeowners can do for the health of their trees and woody ornamentals.  However, when mulch is applied incorrectly, it can adversely affect plants.  Mulch that is too deep, piled against the trunk of the tree, or the use of the wrong material can cause significant harm to trees.  To avoid this potential damage and to maximize the benefits mulch can provide, keep these few key principles in mind:

  • Mulches are available in two primary forms: organic and inorganic. Inorganic mulches include various types of stone, lava rock, pulverized rubber, fabrics, and other materials. Inorganic mulches do not decompose therefore they do not improve soil structure, add organic materials, or provide nutrients to the soil. For these reasons, arborists recommend against inorganic mulches and strongly encourage organic mulches. Organic mulches include wood chips, hardwood and softwood bark, leaves, as well as other products usually derived from plants. Organic mulches decompose in the landscape improving soil quality and fertility. The health of the soil will partially dictate the health of the tree.  Replenishing organic material in the soil profile is necessary for any plant to successfully grow and reach its potential. 
  • Mulch as much of the root zone area as possible. The goal is to maximize the area of soil under the mulch to provide an environment for optimal root growth.  Ideally, mulch should extend to the outermost edge of the canopy, the “drip line.”  At the very minimum, a 3 feet circle around individual trees should receive mulch.  For newly planted trees and shrubs, it is advised to extend the ring by an additional 1ft beyond the outside edge of the root ball.  Keep in mind, the drop line moves out as the tree grows.  Extension of tree rings should occur through the life stages of the tree when room permits.
  • Over mulching landscape trees is all too common. Only 2” to 4” layer of mulch is necessary, in total.  This means 2” – 4” of mulch above the natural soil level.  Because organic mulches decompose, they must be replenished over time.  However, buildup can occur if reapplication outpaces decomposition.  Applying new mulch over old mulch is the same as applying too deep a layer all at once.  Before replenishing the mulch each season, check the existing depth.  Typically, organic mulch requires a refreshening or replacement every 2 – 3 years.  The appearance of old mulch can be “refreshed” by breaking up and fluffing any matted layers by hand or with a rake.  This will not only revive the mulch color to look “fresher” but also encourage better filtration of air and water to the tree roots. 
  • Mulch out, not up. The goal is to keep the tree trunk dry and roots moist.  Allow the root flare (where the trunk meets the soil) to show.  As a general rule for all tree/shrubs, regardless of species, keep mulch at least 3” – 6” away from trunks of young trees and shrubs, and at least 8” -12” away from the trunk of mature trees.  It is important not to place mulch up to the bark of trees or shrubs.  This practice is known as “volcano” mulching.  It is never recommended.  Bark decay can occur when mulch is left against tree trunks or stems of shrubs for extended periods of time.  Mulch causes continuous dampness that encourages decay, attracts insects, fungi, and bacteria to feed on the rotting wood.  Wood rot can develop and cause the eventual death of woody plants.  Piling mulch against the trunk can also lead to girdling roots which can also have a negative impact on the tree’s health. 


Whether it is yourself or a hired contractor that will be accomplishing the task of adding mulch to your landscape, be sure to implement these standards so your trees will receive all the benefits mulch has to offer!

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