University of Illinois Extension

Using Organic Mulches

Organic mulch does much more than add beauty to gardens and foundation plantings, said a University of Illinois Extension horticulture educator.

“Visual appeal is definitely a plus but organic mulch also maintains a more consistent level of soil moisture, modifies soil temperature fluctuations, and aids in weed control,” said James Schuster. “In addition, mulch rings around trees help keep the mowers and string weed trimmers away from the trunk. If the mower and weed trimmer consistently injure the trunk, the tree could die or develop disease and insect problems.”

An organic soil has between four and seven percent organic matter in it, he noted. Good soils have 50 percent of their volume for pore space that contains the necessary air and water. “Continually working organic matter into the ground improves porosity and moisture retention,” Schuster said. “In addition, organic mulch converts to soil. “Inorganic mulches such as lava rock or shredded tires, do not have as many advantages as organic mulches. Inorganic mulches do not conserve moisture or modify soil temperature fluctuation as well as the organic mulches do.” Yard waste can be converted to home-made mulch by partial composting. Homemade mulch may vary in size, color, and texture. For a more uniform appearance, organic mulches may be purchased for consistency in size and color.

“Organic mulches should be applied between two and four inches deep,” he said. “Coarse, large bark may need to be thicker to cut down on light getting through to the soil and encouraging weeds to grow.

“Do not place mulch right against the base of plants. If plant stems do not adequately dry between rains and watering, there is the possibility that the stems will rot and decay. A thick layer of mulch may not only rot herbaceous stems, but also the woody stems of trees and shrubs. ”Schuster said it is best to avoid the volcano effect seen along the highways and now in many home landscapes. Even though there are many benefits from using mulches, there are disadvantages.

“One disadvantage is an increase in insects,” he said. “There are several insects and insect relatives that find organic mulches very desirable to breed and live in. Most of the insects are nuisance insects and, therefore, should be ignored.

“For insects such as earwigs, slugs, and so on, control may be necessary. Contact our local U of I Extension office for recommended controls.”

Organic mulches also decay. On compacted organic mulch, after heavy watering or rains, fruiting bodies such as mushrooms frequently appear. To reduce the appearance of these structures on the mulch, drag a cultivator or garden rake through the mulch as often as needed. More “raking” is needed if mulch is watered and walked on frequently. The decay goes on but the fruiting bodies do not appear.