Tender Plants Injured by 'Sour' or 'Toxic' Mulch

Gail Ruhl, Senior Plant Disease Diagnostician, Department of Botany & Plant Pathology, Purdue University

Burn-like symptoms that appear on plant tissue shortly after applying a hardwood bark mulch around plants may be due to a problem known as "sour" or"toxic" mulch.

Hardwood bark mulch that has been stacked in tall piles for long periods may begin to compost anaerobically (without air). The term 'sour' or 'toxic' refers to the extreme harmful acidity (pH of 1.8-4.0) that can occur under such conditions (in contrast to a "normal" mulch pH around 7.0). Some reports indicate that pine bark does not sour.

Hardwood mulch tends to heat as it breaks down and steam may be seen escaping from the pile when it is disturbed. This heat, by itself, can directly injure plants if the mulch is not allowed to cool first. However, the more serious problem is that chemicals such as ammonia, alcohol, methanol, acetic acid or hydrogen sulfide, produced by fermenting organisms in the mulch pile under oxygen-deficient conditions can cause significant injury on annual and perennial landscape plants.

Injury to young, tender plants is swift; usually within one day of mulch application. Symptoms may include yellowing, bleaching or blackening of foliage and leaf drop. Depending on the extent of the injury, plants often are able to recover. Thorough watering especially during hot, dry weather will help prevent further stress.

The best course of action is to prevent 'sour' mulch injury in the first place. If hardwood bark must be stacked in large piles, turn piles periodically to allow some aeration. Alternatively, hardwood bark mulch that has already soured can be allowed to mellow before use by spreading the mulch in shallow layers and allowing the mulch to air out for at least a few days, preferably a week, before using around young, tender plants. If no rain has occurred, watering the mulch may also help wash away toxic accumulations.

Toxic Mulch - Michigan State University

Image credits:

Figure 1 - http://hortweb.cas.psu.edu/

Figure 2 - http://www.oznet.k-state.edu/

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