Proper Mulching Techniques

Do your trees a favor and mulch them properly!

It is that time of year when we all start focusing on yards and landscaping.  One common misconception is that “lots of mulch around tree trunks is a good thing”.  The opposite can be true if improperly applied.  Improper mulching can do more harm than good to trees.  It has been observed that a number of our street trees are mulched using what is called “the volcano technique”.  That is, the mulch is sloped upwardly from the ground level all the way up to the tree trunk.  This causes the tree to generate additional root growth in the mulch rather than the mulch preserving moisture as it was intended to do.  Please note the following information on proper techniques.  Feel free to share this information from horticultural professionals with any landscaping service company you may be using.

Proper Mulching Techniques

Mulches are materials placed over the soil surface to maintain moisture and improve soil conditions. Mulching is one of the most beneficial things a home owner can do for the health of a tree. Mulch can reduce water loss from the soil, minimize weed competition, and improve soil structure. Properly applied, mulch can give landscapes a handsome, well-groomed appearance. Mulch must be applied properly; if it is too deep, it can actually cause significant harm to trees.

Not Too Much!

As beneficial as mulch is, too much can be harmful. The generally recommended mulching depth is 2 to 4 inches. Unfortunately, many landscapes are falling victim to a plague of over-mulching. A new term, “mulch volcanoes,” has emerged to describe mulch that has been piled up around the base of trees. Most organic mulches must be replenished, but the rate of decomposition varies. Some mulches, such as cypress mulch, remain intact for many years. Top dressing with new mulch annually (often for the sake of refreshing the color) creates a buildup to depths that can be unhealthy. Deep mulch can be effective in suppressing weeds and reducing maintenance, but it often causes additional problems.

Problems Associated with Improper Mulching

  • Deep mulch can lead to excess moisture in the root zone, which can stress the plant and cause root rot.
  • Piling mulch against the trunk or stems of plants can stress stem tissues and may lead to insect and disease problems.
  • Some mulches, especially those containing cut grass, can affect soil pH. Continued use of certain mulches over long periods can lead to micronutrient deficiencies or toxicities.
  • Mulch piled high against the trunks of young trees may create habitats for rodents that chew the bark and can girdle the trees.
  • Thick blankets of fine mulch can become matted and may prevent the penetration of water and air. In addition, a thick layer of fine mulch can become like potting soil and may support weed growth. 

Proper Mulching

It is clear that the choice of mulch and the method of application can be important to the health of landscape plants. The following are some guidelines to use when applying mulch:

  • Inspect plants and soil in the area to be mulched. Determine whether drainage is adequate. Determine whether there are plants that may be affected by the choice of mulch. Most commonly available mulches work well in most landscapes. Some plants may benefit from the use of a slightly acidifying mulch such as pine bark.
  • If mulch is already present, check the depth. Do not add mulch if there is a sufficient layer in place. Rake the old mulch to break up any matted layers and to refresh the appearance. Some landscape maintenance companies spray mulch with a water-soluble, vegetable-based dye to improve the appearance.
  • If mulch is piled against the stems or tree trunks, pull it back several inches so that the base of the trunk and the root crown are exposed.
  • Organic mulches usually are preferred to inorganic materials due to their soil-enhancing properties. If organic mulch is used, it should be well aerated and, preferably, composted. Avoid sour-smelling mulch.
  • Composted wood chips can make good mulch, especially when they contain a blend of leaves, bark, and wood. Fresh wood chips also may be used around established trees and shrubs. Avoid using noncomposted wood chips that have been piled deeply without exposure to oxygen.
  • For well-drained sites, apply a 2- to 4-inch layer of mulch. If there are drainage problems, a thinner layer should be used. Avoid placing mulch against the tree trunks. Place mulch out to the tree’s drip line or beyond.

Remember: If the tree had a say in the matter, its entire root system (which usually extends well beyond the drip line) would be mulched. 

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