Gardening resources > Soils and composting > Soils and composting factsheets > Mulches for landscaping > Beware of toxic mulch
Cornell University Department of Horticulture
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Cornell gardening resources Beware of toxic mulch

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  • Mulching is a great way to suppress weeds and conserve moisture. But toxic liquids and gasses can form in improperly prepared or stored wood-chip and bark mulch.

    They can kill your plants if you're not careful.

    The best way to prevent this problem is to smell your mulch before applying it. If it has a pungent odor similar to vinegar, rotten eggs or silage, don't spread it around your plants. Chances are good you've got "toxic" or "sour" mulch.

    If the mulch smells like freshly cut wood or fertile garden compost, chances are slim that it will be a problem.

    The toxins in sour mulch dissipate rapidly. If your smell test leads you to suspect a problem, spread the mulch out in a shallow layer for a few days on a driveway, tarp, or other place where it won't damage plants. Exposure to air will usually get rid of the gaseous toxins. If the weather is dry, you can water the pile to leach out liquid toxins.

    Have your mulch delivered well ahead of when you need it so that you don't have to rush to apply mulch that could benefit from a few days or weeks of curing.

    Once you've spread sour mulch on your plants, there isn't much you can do. They may recover on their own after being set back some. But removing the mulch seldom helps once the damage is done. Herbaceous plants and newly planted trees and shrubs are most susceptible.

    The symptoms of toxic mulch usually occur within 24 hours of application. The damage resembles that caused by drought, poor drainage, fertilizer burn, or pesticide misapplication. Symptoms include yellowing of leaf edges, scorched-looking leaves, defoliation, and/or death of plants.

    Sour mulch problems usually start where the mulch is made. Wood chips should be stored in long, low (4- to 6-foot tall) windrows and should be turned frequently. If they aren't, pockets in the center of the pile may not get enough oxygen. The mulch may become laden with toxic byproducts such as methanol, acetic acid, ammonia and hydrogen sulfide gas. The mulch may also become very acidic, with a pH in the range of 1.8 to 3.6. (Properly composted organic material has a near-neutral pH of 6.0 to 7.2.)

    In general, hardwood chip mulch is more of a problem than pine bark mulch. And while bulk mulch sources are more prone to be sour, bagged wood chips should be suspect, particularly if the bags lack air holes and the mulch is excessively wet. Again, use your nose to check for off smells.

    Toxic mulch problems are not limited to commercial sources. Large piles of chips from storm damage clean-up or other tree trimming can also develop sour pockets. Again, the remedy is to turn the pile and expose the sour mulch to air and rain.

    Special thanks to Joel W. Allen, Cooperative Extension Educator, Columbia County, for his help.

    Copyright, Department of Horticulture, Cornell University.

    Website design: Craig Cramer

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