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Sour Mulch Burns Tender Plants

B. Rosie Lerner, Extension Consumer Horticulture Specialist

We've had several calls recently from gardeners and landscapers that have experienced damage to young flower plants shortly after applying a hardwood bark mulch. Symptoms appear as if the plants have been burned with fertilizer or pesticides, or possibly like severe water stress. All of the above could potentially be a problem, but apparently, we must now add "sour mulch" to the list of suspects.

When hardwood bark mulch has been stacked in tall piles that are allowed to stand for long periods, the material begins to compost anaerobically (without air). The term "sour" refers to the extreme acidity that occurs under such conditions (reported to be pH 1.8 - 3.6, while that of "normal" mulch is generally close to 7.0. Some reports indicate that pine bark does not sour.

The mulch tends to heat as it breaks down and steam may be seen escaping from the pile when it is finally disturbed. This heat can directly injure plants if the mulch is not allowed to cool first. However, the more serious problem is that sour mulch leads to the production of several chemical components possibly including methane, alcohol, ammonia, and hydrogen sulfide.

Injury to young, tender plants is swift; usually within one day of mulch application. Symptoms include yellowing 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. Do not apply fertilizer to plants injured by sour mulch; such materials can cause further injury. If plants recover and seem to be lacking in vigor, a side-dressing of nitrogen may be called for in mid-summer.

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 them periodically to allow some aeration. Alternatively, hardwood bark mulch that has already soured can be allowed to mellow before use. Spread the mulch in shallow layers or apply to the beds to be mulched. Allow 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.

The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by the Purdue University Cooperative Extension Service is implied. Any person using products listed assumes full responsibility for their use in accordance with current direction of the manufacturer. Purdue University is an equal opportunity/equal access institution.

Information listed is valid only for the state of Indiana.

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