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This is first in a two-part series on mulching. Part two, on mulching recommendations, will appear in our February issue.

Chris Carlson is Director, Horticulture Technology, and Associate Professor at Kent State University, Salem Campus, Salem OH. Photo credits: Chris Carlson.

The proper choice and application of mulch can dramatically improve the overall health and vitality of trees and other landscape plants. Improper mulching, on the other hand, can needlessly stress and potentially kill plants. Arborists should be aware of the benefits of mulch, the characteristics of various types of mulch, and the potential problems associated with over-mulching.

Piling soil or mulch, or both, against the trunk flare has been shown to prevent needed gas (CO2 and O2) exchange, causing phloem stress, dysfunction, and subsequent root stress.

The two basic mulch choices are inorganic and organic. Inorganic mulches such as lava rock, mineral rock and gravel, pulverized rubber, geotextile fabrics, and other materials do not decompose like organic mulches, which is why many contractors choose to use them. They do not need to be replenished often, but neither do they contribute to the organic matter in the soil. Organic mulches usually are derived from plants or plant parts. Examples include cocoa hulls, conifer needles, leaves, grass, newspaper, straw, hardwood and softwood (conifer) bark, wood chips, and other wood products. Organic mulches generally have more effect on soil structure and microbial activity than inorganics do. The fact that they decompose means that their longevity in the landscape is shorter than that of inorganics.

Even among organic mulches, longevity is variable. Wood decomposes more quickly than bark, conifers (softwoods) more quickly than hardwoods, fine mulch more quickly than coarse, succulent tissues more quickly than woody materials, and fresh tissue more quickly than dry. Conifer bark nuggets from large, mature pine, cypress, or other softwood trees contain very high amounts of lignin, wax, and protected cellulose that resist decay. Wood from these same species, however, rots quickly because the cellulose (in young trees especially) is not yet protected by the production of lignin and other materials that resist decay.

Hardwood tree bark, even from large trees, contains large amounts of cellulose that is not protected from rotting. Wood and hardwood bark have high carbon content and low nitrogen content (that is, high carbon to nitrogen ratios, C:N). Microorganisms that decompose wood use nitrogen in the decomposition process, which is why plants sometimes exhibit temporary nitrogen deficiencies. This phenomenon is especially noted when high wood content or hardwood bark mulches with particles smaller than 3/8 inch are incorporated into soil planted with herbaceous and woody ornamentals. Because of this nitrogen immobilization, many mulch producers screen out all mulch particles less than 3/8 inch to prevent this problem. Other producers additionally compost their mulch for a minimum of six weeks and add 1 to 3 pounds of actual nitrogen per cubic yard of mulch to speed composting and lower the carbon to nitrogen ratios to the ideal twenty parts carbon to one part nitrogen. This composting procedure kills plant disease pathogens and eggs of insect pests, and it produces a product that returns plant nutrients rather than ties them up.