Temperature and moisture changes often cause mold to form on hardwood mulch. These masses, which include slime molds and toadstools, are usually not toxic, but they can be unsightly. Take steps when laying the mulch to keep mold from forming. Additionally, if mold has already sprung up on the hardwood mulch, you can attempt to get rid of it by turning or removing the mulch, but it's usually more effective to avoid it from the start.
Lay mulch in beds no deeper than 1 1/2 to 2 inches. Deep piles of mulch encourage mold growth because they decompose during the hot summer months, leaving a dried-out pile of wood behind. Mold forms easily on dry mulch.
Choose a hardwood mulch that is not finely ground. Hardwood mulch is particularly prone to mold issues because it rots very quickly. The larger the wood chunks are, the slower they will decompose. This makes it easier to keep the moisture level in balance.
Soak the mulch with water as it is applied to the area. When mulch is first laid down, it comes from large, self-heating piles that often hit an internal temperature of 140 to 160 degrees Fahrenheit. This creates a prohibitive environment for mold growth. If the heated mulch is watered, bacteria can flourish. As the hardwood mulch cools, the bacteria remain and compete with the mold for resources. This can keep mold issues at bay.
Keep the mulch damp, watering it several times a week if necessary during the hot summer months.
Turn the mulch into the underlying dirt with a shovel and garden rake if mold develops on its surface. Soak the turned mulch with water.
Remove all the mulch from the area if the mold continues to grow. Wet it down thoroughly, then shovel it into a pile. The self-heating that naturally occurs when the hardwood mulch is piled will kill any remaining mold.
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