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Wood Chips
Mulch
March 23, 2006

 

(photograph courtesy of Sue Auerbach)

Trees and other plants growing in natural settings, such as woodlands, do best with the natural mulch that occurs when branches and leaves fall and decompose on the forest floor. These decaying natural materials replenish nutrients and provide an optimal environment for root growth and mineral uptake.

Systematic mulching in other areas, when done correctly, can greatly enhance the health and appearance of your trees, shrubs, and other plantings, preserve water resources, and reduce the cost and effort of controlling unwanted vegetation. However, how that mulching is done is very important. There is a right way and a wrong way to mulch, particularly around trees. Proper mulching can be beneficial, whereas improper mulching can severely damage and expose trees to insects and disease.

What is mulch?
Mulch is a protective layer of material on the soil surface adjacent to plants. Mulch may be organic (derived from plant material) or inorganic (derived from non-plant materials), and may be in particle or sheet form.

Why mulch?
Mulch serves many purposes, including:

  1. Inhibiting weed growth
  2. Stabilizing soil temperature by keeping it warmer in cold weather and cooler in hot weather.
  3. Maintaining moisture in the soil.
  4. Reducing water run-off and soil erosion.
  5. Reducing soil-borne plant diseases.
  6. Reducing soil heaving from the freeze-thaw cycle, which breaks plant roots.
  7. Protecting plant roots from soil compaction.
  8. Improving appearance of planted areas (obviously a subjective valuation).
  9. In the case of organic mulches, enriching the soil with nutrients through decomposition.
  10. Research by Weyerhaeuser Co. suggests that a two inch layer of bark mulch reduces summer moisture loss by about 20%, and reduces soil temperature in the upper four inches by about 10 degrees F.

How thick?
Always avoid the “volcano” and “telephone pole” effects ­ piling mulch like a volcano around a tree so that the trunk projects like a telephone pole from the mulch pile. A 2 to 3 inch depth is generally recommended. Shallower applications may not accomplish the above objectives. Heavier applications may encourage weed growth and may lead to overly-wet soil and rotting roots, especially with the heavy clay soil found in many parts of Riverwoods. Ideally, mulching around trees extends to the drip line, since tree roots generally extend at least that far.

Never apply mulch against the trunk of trees or the stems of woody plants. Such an application may prevent the trunk and stems from properly drying, leading to disease. It may also promote undesirable shallow root growth within the mulch, and encourage rodents that burrow into deep mulch and chew on wet tree trunks.

Don’t forget that tree roots are very shallow ­ within the upper 6 to 12 inches of soil. Heavy applications of mulch, especially very fine-particle mulch, can smother tree roots.

When to apply?
The best time to apply mulch is in the mid-to-late spring or early summer, well after the soil has thawed and when it is moist but not soggy. Late fall mulching, after the first frost, may also be beneficial to reduce soil heaving.

Which mulch?
Among the considerations are aesthetics, durability, soil type, mulch availability and cost. Inorganic mulches tend to be more long-lasting than organic mulches, since the former decompose slowly if at all. In addition, they do not burn as readily as organic mulches. Organic mulches absorb water more effectively than inorganic mulches. They decompose, and have to be replaced periodically ­ generally at least annually. Through decomposition they provide valuable nutrients to the soil.

Organic mulches are not, however, fertilizer substitutes. The microorganisms responsible for decomposition of organic mulches such as wood chips require nitrogen for the decomposition process. The decomposition process may deplete the soil of nitrogen, stunting growth and turning leaves yellow, unless a nitrogen-containing fertilizer is added to the mulch. It is generally suggested to apply fertilizer containing 10% nitrogen at the rate of about 1 pound per 100 square feet.

The pH of the soil should also be considered, since many plants are pH-sensitive. Composted mulch tends to be slightly alkaline, while mulch composed of oak leaves, pine needles and/or peat moss tends to be slightly acidic.

Organic mulches
These mulches decompose, generally providing nutrients to the soil, and must be replaced periodically. Wood chips. Wood chips are readily available, frequently without cost, from landscapers, arborists and Commonwealth Edison when its trucks are trimming trees that may interfere with utility lines.

Wood Chips Wood chips decompose rapidly, and need to be supplemented with nitrogen-containing fertilizer. Walnut wood chips should be avoided because they are toxic (called “allelopathy”).
Bark Shredded or chipped bark is available commercially. It decomposes more slowly than wood chips, and some people find it more pleasing aesthetically.
Pine needles Pine needles and pine cones are of limited availability. This type of mulch is best for plants that require acidic soil.
Peat moss In general, peat moss is better as a soil amendment than mulch. While there are different kinds of peat moss, sphagnum peat moss is the most common and probably the most useful mulch. It is somewhat acidic, and decomposes slowly. Peat moss is also made from swamp plants, such as reeds and cattails. It is more alkaline, and decomposes more quickly.
Leaves Dry and ground leaves are suitable for mulch, especially if partially decomposed. Wet, matted leaves should be avoided, since they tend to divert water rather than absorb it. Maple leaves are alkaline and oak leaves are acidic. Walnut leaves should be avoided because they contain toxins.
Grass clippings Grass clippings are suitable for mulch unless herbicides have been used. Fresh grass clippings, especially if applied thickly, can generate excessive heat. It is best to first compost grass clippings with leaves, to begin the decomposition process prior to use.
Manure Manure should be composted prior to use, to avoid burning plants. Composted manure adds essential nutrients to the soil, and is more useful as a soil amendment than mulch.
Straw Straw should be supplemented with a nitrogen-containing fertilizer. Because straw contains seeds, thick applications may attract rodents.
Hulls Buckwheat, cottonseed and cocoa hulls are decorative, but expensive. They also tend to wash away easily.

Inorganic mulches
You need to be sure you like these mulches, since they do not decompose. They are long-lasting, but provide no nutrients to the soil.

Stone and Pebbles These materials are used for decorative effect ­ color and texture. Unless used over sheeting material, they may tend to get mixed with soil over time. Limestone mulches are alkaline.
Recycled rubber tires Because of the large volume of discarded tires, alternatives to landfill disposal are being encouraged. Shredded tires may be a suitable mulch, although there is some concern about flammability and chemicals content.
Plastic sheeting Plastic sheeting (typically black plastic) is inexpensive, but becomes brittle if exposed to the sun. Plastics are not advisable for long-term use since they are impermeable to water and air, either leaving the soil too moist or preventing rainwater from reaching the soil.
Landscape fabric Landscape fabrics (sometimes called geotextiles), which are usually woven or perforated plastics, solve some of the problems of solid plastic sheeting, because the former are more durable and allow passage of water and air. Their tiny perforations may become clogged with silt over time. To avoid eventual breakdown by sunlight, they should be covered with another mulch material.