Mulching Your Trees and Landscapes

By Wayne K. Clatterbuck, Dept. of Forestry, Wildlife & Fisheries, University of Tennessee; and International Society of Arboriculture

What Is Mulch?

A protective covering of various materials, either organic or inorganic, spread over the soil surface to reduce evaporation of moisture, improve plant growth, discourage weedy species and enhance the appearance of the landscape is known as mulch.

Different types of mulches: (a) wood chips, (b) pine bark nuggets, (c) shredded hardwood bark and (d) brick chips. 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. Photos by Wayne Clatterbuck

Inorganic mulching materials include landscape fabric (geotextile) and plastic film that are primarily used for weed control and retention of soil moisture, while rocks, gravel, brick chips and shredded rubber are used more for decorative purposes. Most inorganic mulches do not decay and thus do not improve soil properties. Grass clippings, straw, pine needles, leaves, peat moss, compost, wood fiber and bark are organic mulches composed of decaying plant materials. Over time, organic mulches will decompose and become part of the soil, adding to the soil?EUR??,,????'???s organic matter, improving soil structure and nutrient availability and helping the soil retain moisture. However, as the organic mulches decompose, they must be replenished.

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 or if the wrong material is used, it can actually cause significant harm to trees and other landscape plants.

 

Mulch should not be more that three to four inches deep. In this example, the mulch is much too deep, about 18 inches. Deep mulch can lead to excess moisture in the root zone, which can stress the plant and cause root rot.

Types of Mulch

Mulches are available commercially in many forms. The two major types of mulch are inorganic and organic. Inorganic mulches include various types of stone, lava rock, pulverized rubber, geotextile fabrics and other materials. Inorganic mulches do not decompose and do not need to be replenished often. On the other hand, they do not improve soil structure, add organic materials, or provide nutrients. For these reasons, most horticulturists and arborists prefer organic mulches.

Organic mulches include wood chips, pine needles, hardwood and softwood bark, cocoa hulls, leaves, compost mixes and a variety of other products usually derived from plants. Organic mulches decompose in the landscape at different rates depending on the material. Those that decompose faster must be replenished more often. Because the decomposition process improves soil quality and fertility, many arborists and landscape contractors consider that characteristic a positive one, despite the added maintenance.

Crushed brick is an inorganic mulch that is decorative and long-lasting, being used on the median of a roadway.

Benefits of Mulch

?EUR??,,????'???? Moderates the temperature of the root zone. Mulch provides an insulation effect, keeping the soil warmer during the winter and cooler during the summer.

?EUR??,,????'???? Encourages the conservation of moisture in the soil by reducing surface evaporation.

?EUR??,,????'???? Helps control weeds. A 2 to 4 inch layer of mulch reduces the germination and growth of weeds.

?EUR??,,????'???? Prevents the soil from crusting over, allowing water to penetrate and percolate. Mulch can improve soil aeration, soil structure (less soil compaction) and drainage.

?EUR??,,????'???? Adds organic matter to the soil, improving fertility.

?EUR??,,????'???? Restrains soil erosion, especially raindrop erosion.

?EUR??,,????'???? Reduces the chance of mechanical injury to trees and shrubs from lawn mowers and weed trimmers.

?EUR??,,????'???? Gives the landscape a more pleasing, manicured appearance.

Trees growing in a natural forest environment have their roots anchored in a rich, well-aerated soil full of essential nutrients. The soil is blanketed by leaves and organic materials that replenish nutrients and provide an optimal environment for root growth and mineral uptake. Urban landscapes, however, are typically a much harsher environment with poor soils, little organic matter and large fluctuations in temperature and moisture.

Volcano mulching can have a harmful effect on plant roots and the trunk of the tree. Mulch piled high against the trunks of young trees may create habitats for rodents that chew the bark and can girdle the trees.

The root system of a tree is not a mirror image of the top. The roots of most trees can extend out a significant distance from the tree trunk. Although the guideline for many maintenance practices is the drip line?EUR??,,????'??+the outermost extension of the canopy?EUR??,,????'??+the roots can grow many times that distance. In addition, most of the fine, absorbing roots are located within inches of the soil surface. These roots, which are essential for taking up water and minerals, require oxygen to survive. A thin layer of mulch, applied as broadly as practical, can improve the soil structure, oxygen levels, temperature and moisture availability where these roots grow.

Improper Use of Mulch

Even with the many benefits of mulch, improper application can have a detrimental effect on trees. Most plants are sensitive to the depth of mulch. More than four inches of mulch restricts soil oxygen exchange with the roots. The effect is similar to planting trees deeper than they were grown in the nursery. Roots will have a tendency to grow upward into the mulch to gain better access to oxygen. Thus, instead of the roots spreading outward throughout the soil, they will congregate in the mulch. Excessive mulch can lead to accumulation of moisture in the root zone, as well as moisture deficits during droughts, making roots more susceptible to rots, insects and disease.

A common practice in many landscapes is volcano mulching ?EUR??,,????'??? the placing of mulch in a cone around and next to the trunk 8 to 12 inches high and extending horizontally about 2 feet in diameter. This depth of mulch restricts oxygen exchange with the roots, but also creates a moist environment at an elevated position near the base of the stem. This moist environment promotes cracking of the bark, creating an entry point for insects and fungal growth as well as conditions favorable for rodents to chew the bark and damage or girdle the tree.

Use of mulch that has not been properly composted or aged can create problems for many plants. Fresh grass clippings produce heat as they decay that may injure young plants. Other mulches, particularly fresh sawdust or wood wastes, do not contain adequate nitrogen for micro-organisms to decompose them. Decomposition of these mulches will remove nitrogen from the soil unless additional nitrogen is supplied, usually 1/2 to 1 pound of ammonium nitrate per 100 square feet of mulched area (Sams 1998). Mulch can also change the pH of the soil, influencing nutrient availability. Some materials, such as pine needles and oak leaves, are more acid, while grass clippings and hardwood bark are more alkaline. Anaerobic or ?EUR??,,????'??sour?EUR??,,????'?? mulch in materials that have not been properly composted have pungent odors that build from the production alcohols and organic acids. These compounds can be phytotoxic to young plants and should be avoided by using composted materials rather than fresh mulch.

Mulch provides definition to a well-manicured and attractive landscape. Note that the mulched area extends beyond the canopy of the tree.

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 non-composted wood chips that have been piled deeply without exposure to oxygen.

Mulch wide, not deep. 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.

Summary

Deciding which mulch is best for you will depend on your purpose, preferences and associated costs. Before choosing a mulch, consider how you will use it. Organic mulches are more natural and enhance many soil properties, while inorganic mulches are used more for decorative purposes.

Both kinds of mulch conserve soil moisture to some extent and provide some weed control. All mulches have different properties that may be advantageous in some situations and detrimental in others. Make sure that mulch is applied correctly to enhance tree growth and that cured or aged materials are used.