Horticulture News: The good, bad and ugly of mulching

By Adam Leonberger, Published:

Mulching offers many benefits for landscaping in your yard and gardens.  Some of these include retained moisture, weed control, improved drainage, lower soil temperature, erosion prevention and protection from mowers and trimmers. As they decompose, mulches also release minerals into the soil and leave behind humus, which is good for plants.  

Generally, these organic materials inhibit undesirable microorganisms such as soil borne pathogens that cause diseases of plants. They also stimulate the activity of many types of beneficial microorganisms, including mycorrhizal fungi.

Wet and warm weather can cause mulch to produce some undesirable consequences. The combination of seasonal rains and fresh wood chip or bark mulch can result in the proliferation of nuisance fungi on the mulch surface. Gardeners most often spread mulch in spring and fall, so being aware of these potential problems and how to lessen them is beneficial. 

Harmless or nuisance?

The following information from Dr. John Hartman, extension plant pathologist, will give you clues to some of these fungi you may see in your home landscape. 

Some forms of fungi, like stinkhorns, bird’s nest fungus, earth stars, toadstools and slime molds are unattractive but fairly harmless. Other types, like the shotgun or artillery fungus are truly a nuisance.

The artillery fungus shoots tiny masses of black spores onto nearby surfaces like home siding and cars. When fungus mycelium permeates a thick layer of dry mulch it can block water saturation and cause irrigation problems.  

Hardwood mulches, especially when they are finely ground, contain a large amount of cellulose that decomposes rapidly and leads to nuisance fungi. Composting this type of mulch will result in the growth of competing bacteria and other molds that can inhibit the development of fungi.  

How to control fungi

Gardeners who want to avoid unsightly growths on their mulch can purchase composted mulch products. Mulches low in wood content and high in bark content are less vulnerable to fungi.

Finely ground wood products should be avoided unless they are composted first. Fresh wood chips need to be wetted down and have nitrogen fertilizer applied if they don’t contain fresh leaves. Compost this mixture for six weeks before applying to landscaping.  

Sometimes very little can be done to control nuisance fungi other than to spade the mulch into the surface soil layer followed by soaking with water. 

Another option is to remove the mulch and place it in a heap after thorough wetting to allow for self-heating to occur at 110-140 degrees Fahrenheit. This will kill nuisance fungi. If fresh dry mulch is placed on top of mulch colonized by nuisance fungi, the problems may occur again the following year or even earlier. 

Mulch should be applied no more than two inches in depth and should be soaked with water immediately after application to enhance the colonization of beneficial bacteria. Avoid using or purchasing sour mulches with an acrid odor. They are highly acidic and can injure plants.

Photos and additional information can be found on the UKY publication “HO-106 Mulch Myths” at: http://www2.ca.uky.edu/agc/pubs/ho/ho106/ho106.pdf. For details, contact the Franklin County Cooperative Extension Service at 502-695-9035 or email Adam.Leonberger@uky.edu.

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