Is it safe to use mulch made of shredded tires? —Julie Holt, Interlachen, FL
Major building-supply stores sell garden mulch made from shredded tires. It’s expensive, but it’s long lasting, allows for good moisture drainage, and is available in various colors. But all shredded-tire mulches are not the same. When made from newer tires (since the mid-1970s), they may have steel-belt reinforcements shredded with the rubber, though some costlier versions claim all steel particles are removed. This steel is coated with zinc to make it rustproof. Depending on how much of the steel belting has been removed, the mulch might leach small amounts of zinc, manganese, and iron into the soil.
Leaching is more pronounced in acidic soils than in alkaline soils. And there is more potential for problems in soils that lack organic materials. If the soil has high organic content, those leached metals become bound tightly into the soil, rendering them unavailable to plants.
Is it harmful to use shredded tires? There isn’t any scientific research to give us a definitive answer. I doubt the shredded tires would be harmful to most plants or garden wildlife, but plants that are sensitive to excess zinc—such as tomatoes, cabbage, beans, and peanuts—might have trouble in sandy, acidic soil. If you’re concerned about using shredded tires, there are plenty of other mulching materials from which to choose.