Mulch Not Just for Looks

 

By Sonia Abney Garth, marketing coordinator, International Society of Arboriculture; and Sharon Lilly, ISA board certified master arborist.






Mulch isn’t just shredded plant materials these days. Rock, crushed stone, even nut shells and shredded rubber can be successfully employed as mulch.


Many contractors choose mulch because of the well-cared-for look it gives a landscape.

With mulch the result can be a better growing environment for trees and their roots. Landscapers should be aware that, generally, the root system of a tree spreads out, not down. “The roots of most trees extend out a significant distance from the trunk. Most of the fine absorbing roots of trees are located within inches of the soil surface,” says Jim Skiera, executive director of the International Society of Arboriculture. These shallow roots are essential for taking up water and minerals for trees, and they require oxygen to survive. A thin layer of mulch, spread widely, can provide a healthier environment where these roots grow.

Mulch Benefits

Properly applied mulch provides many benefits to the health of a tree. Unlike trees growing in a forested environment, urban trees are not typically planted in an optimal environment for root growth and mineral uptake. Typically, urban environments are harsher with poor soil conditions and large fluctuations in moisture and temperature. Applying mulch can help reduce the stress of such conditions through these benefits: helping to maintain soil moisture with less evaporation; reducing the number of weeds; providing insulation by keeping soil cooler in the summer and warmer in the winter; protecting from damage caused by lawn equipment such as weed-eaters and lawn mowers; improving soil fertility, aeration and drainage.








Various types of stone and rock mulch provide an attractive landscaping that requires less maintenance. Be careful though when using string trimmers in these areas.







A blanket of mulch can help maintain soil moisture under a small grove of trees.


Organic or Inorganic

Mulches are either organic or inorganic material mixtures that are placed over the soil surface around the base of a tree. Mixtures consisting of various types of stone, rock, pulverized rubber and other materials are labeled as inorganic. Because these types of mixtures do not decompose, they need replenishing less often. However, this also means they do not improve soil structure, provide nutrients, or add organic materials to the soil. Inorganic mulches still provide other benefits such as insulation, and protection. Mulches are placed on the soil surface to reduce moisture evaporation and improve soil conditions. Mulching the soil surface has several benefits. A layer of mulch reduces water evaporation from the soil. It minimizes weed competition (and thus water use), reduces soil erosion and can improve soil aeration. The soil will be kept cooler in the summer and warmer in the winter. In addition, most landscapers and home owners think that a mulched planting bed has a neat, finished appearance. Also, trees that are mulched are less likely to fall victim to lawn mower damage and will develop a more extensive root system. Organic mulches consist of wood chips, pine needles, bark, leaves and other products derived from plants. These mulches decompose, and are beneficial in improving soil quality by replenishing nutrients. They do, however, require more maintenance because decomposition creates the need to replenish more often.



Organic mulches such as wood chips or nutshells are usually preferred because they decompose; improving soil quality and fertility.







Large mulching beds such as this allow for the entire root system to be blanketed providing maximum benefits.


Mulching Dos and Don’ts

In order for mulch to be beneficial, it must be applied correctly. “All things in moderation should be a homeowner’s mulching motto,” says Skiera. “As beneficial as mulch is, too much can be harmful in more ways than one.” Too much mulch can create excess moisture that may lead to root rot. Other problems created by over-mulching include insect and disease problems, weed growth, sour smelling planting beds and chewing rodents. To ensure the health of your trees and plants, follow these practical mulching tips to landscape like the professionals: Thin is better. Apply a two to four-inch layer of mulch, unless drainage problems exist–then a thinner layer is recommended. Do not add mulch if there is already a sufficient layer. Instead, rake the old mulch to break up any matted layers and refresh the appearance. No volcano mulching. Avoid placing mulch against the tree trunks. If mulch is already 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. Mulch out to the tree’s drip line or beyond if possible.

>Mulching: Deeper Isn’t Better

Mulches are materials placed over the soil surface to maintain moisture and improve soil conditions. Mulching is one of the most beneficial things anyone can do for the health of a tree. The application of 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. However, if mulch is applied improperly (too deep or use of the wrong material) it can actually cause significant harm to trees and other landscape plants. 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 harsher environment with poor soils, little organic matter and large fluctuations in temperature and moisture. Applying a two- to four-inch layer of organic mulch can mimic a more natural environment and improve plant health. 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—the outermost extension of the canopy—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.

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 other landscape professionals consider that characteristic a positive one, despite the added maintenance.



Various types of inorganic mulches such as stone and lava rock are often used.


Not Too Much!

As beneficial as mulch is, too much can be harmful. The generally recommended mulching depth is two to four inches. Unfortunately, North American landscapes are falling victim to a plague of overmulching. 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.






If mulch is piled against a tree trunk, pull it back several inches to expose the base of the trunk.







Mulch volcanos are detrimental for the tree. A two to four-inch layer of mulch is all that is necessary.


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 two to four-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!






Thick blankets of mulch can become matted and may prevent air and water penetration.







A wide and thin layer of mulch is healthier for the tree. This mulch has been correctly applied without touching the trunk.


Benefits of Proper Mulching

  • Helps maintain soil moisture. Evaporation is reduced and the need for watering can be minimized.
  • Helps control weeds. A two- to four-inch layer of mulch will reduce the germination and growth of weeds.
  • Mulch serves as nature’s insulating blanket. Mulch keeps soils warmer in the winter and cooler in the summer.
  • Many types of mulch can improve soil aeration, structure (aggregation of soil particles), and drainage over time.
  • Some mulches can improve soil fertility.
  • A layer of mulch can inhibit certain plant diseases.
  • Mulching around trees helps facilitate maintenance and can reduce the likelihood of damage from weed whackers or the dreaded “lawn mower blight.”
  • Mulch can give planting beds a uniform, well-cared-for look.







Mulches, such as this ring of pulverized rubber, can help protect trees from damage caused by lawn mowers or weed trimmers.


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.
  • Anaerobic “sour” mulch may give off pungent odors, and the alcohols and organic acids that build up may be toxic to young plants.

“The roots of most trees extend out a significant distance from the trunk. Most of the fine absorbing roots of trees are located within inches of the soil surface.” —Jim Skiera, Executive Director of the International Society of Arboriculture