Toxins in Mulch – Allelopathic Phytochemicals
By George Giltner, MS, Adv. Master Gardener
Select your choice of mulch wisely! Remember that when leaf, bark, wood, nut stem or root mulch is placed around vegetation, you are essentially placing organic chemicals within the living space of growing plants. When many of these compounds are not decomposed, they perform their original function as a competition inhibitor or toxin. Think of plants as competitors in a continuous battle for nutrients, water, sunlight, and space. “Better living through chemistry” is how they win.
Pliny the Elder, a roman scholar, observed that walnut trees were toxic to other plants. As history repeats itself, gardeners have experienced the effect of juglone, the chemical allelopathic compound of walnut trees that is responsible for reduced growth or death of surrounding plants. Juglone is concentrated in the buds, nut hulls, and roots, but it is also present in leaves and other plant parts.
Tomatoes, peppers, and other Solanaceous plants are very susceptible to juglone’s effect as a respiration inhibitor. The plants will exhibit symptoms as wilting, yellowing, and eventual death. Plants that are sensitive to juglone include apple, azalea, blackberry, blueberry, chrysanthemum, pine, potato, rhododendron, thyme, and many others. However plants that are resistant to juglone include beets, carrots, corn, snap beans, melons, onions, etc. Yet these plants may exhibit some degree of toxicity.
Trees related to walnuts, such as hickories, pecans, and English walnuts also produce juglone, but in smaller quantities. These trees are responsible for pollen allergies in humans and horses. Horses may even be affected by walnut wood chips when it is used as a bedding material.
Aerobic composting of leaves is effective in degrading juglone and other allelochemicals. Moisture, mixing, temperature, and microbial action are factors that determine the degree of decomposition to non-toxic levels which can occur in as little as three weeks. However, it would be safer to allow 6 months of complete decomposition time before using this compost. Also maintain high organic matter around plants to produce microbial populations that can metabolize toxins. Twigs, chips, and sawdust from walnut trees are harder to digest, therefore it is best to avoid using them for mulch, compost, or bedding material.
Another allelotoxin is ailanthone from the ‘Tree-Of-Heaven’, Ailanthus altissima. This tree plant toxin has potent post-emergence herbicidal activity and poses a serious weed problem in urban areas. Sorghum produces sorgolene in most species which disrupts photosynthesis. Therefore it is being extensively researched as a weed suppressant. There are many other allelopathic species which include grasses like Rice, Tall Fescue, some Perennial Rye, woody plants as Cherry, Sycamore, Rhododenderon, Elderberry, Fragrant Sumac, and even Pea (Pisum sativum), Goldenrod, that have allelotoxins.
One way to test for allelopathy is to grow seeds in potentially toxic mulches, compost, or soils. Use side by side control pots with “clean” soil as a control. Over time observe germination, growth rate, length of stem and roots, color of leaves, etc. for any sign of toxicity. “Use What Works” to avoid problems!