I’ve heard mulches made from ground-up tires could be toxic. Is rubber mulch safe to put on my garden beds?
Rubber mulches made from shredded tires are touted by manufacturers as permanent, aesthetically pleasing, and safe for flowers, plants and pets. Companies assert that the mulch material is an environmentally friendly solution to a major waste-disposal problem. But scientific literature makes abundantly clear that rubber should not be used as a landscape amendment or mulch. There is no question that toxic substances leach from rubber as it degrades, contaminating soil, plants and waterways.
The toxicity of rubber leachate is mainly a result of its mineral content: Aluminum, cadmium, chromium, copper, iron, magnesium, manganese, molybdenum, selenium, sulfur and zinc have all been identified in laboratory and field leachates. Of those minerals, rubber contains very high levels of zinc — as much as 2 percent of the tire mass. If rubber products have been exposed to contaminants, such as lead or other heavy metals, during their useful lifetime, they will absorb those metals and release them as well. A number of plant species have been shown to accumulate abnormally high levels of zinc, sometimes to the point of death. U.S. Department of Agriculture researcher Rufus Chaney, who has studied zinc and other metals in soils and plant materials for decades, strongly believes that ground rubber should not be used in any compost, potting medium, or agricultural or garden soils because of zinc toxicity. Acidic soils and aquatic systems are particularly sensitive because heavy metals are less tightly bound to the soil and thus more available for plants and animals to uptake.
Rubber mulches can also leach various plasticizers and accelerators that are used during tire manufacturing. In high enough concentrations, some of these rubber leachates are known to be harmful to human health; effects of exposure range from skin and eye irritation to major organ damage and even death. Long-term exposure can lead to neurological damage, cancers and mutations.
Some of the toxic materials in rubber break down quickly, while others bioaccumulate. One common rubber leachate is 2-mercaptobenzothiazole, a chemical commonly used as an accelerator during the production process. In addition to its known human health effects, it is highly persistent in the environment and acutely toxic to aquatic organisms — its environmental persistence may cause long-term damage to aquatic environments constantly exposed to rubber leachates.
Another family of organic leachates under scrutiny are the polyaromatic hydrocarbons (PAHs). These compounds, used as rubber softeners and fillers, have been repeatedly demonstrated to poison aquatic life. After two years in a laboratory test, PAH leachates were shown to be even more toxic than at the study’s inception.
According to the Environmental Protection Agency, we generate 290 million scrap tires annually, and scrap tire stockpiles pose significant fire hazards. Obviously, we need to find a safe way to recycle these slow-to-decompose materials, but using shredded tires as mulch to spread over our gardens and playgrounds is not a wise choice. A better alternative is to increase the use of shredded tire rubber in asphalt for roadways.
Linda Chalker-Scott is an Associate Professor and Extension Urban Horticulturist at Washington State University’s Puyallup Research and Extension Center. Read more of her work on the blogs The Informed Gardener and The Garden Professors. You can also find The Garden Professors on Facebook.