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Good Growing

Contaminated Landscape Materials: Rubber Mulch

Despite our best intentions to create healthy gardens and landscapes, sometimes we wind up introducing a material that has the potential to affect environmental or human health. Do you know if you have any in your yard? Let’s look at a material commonly found in the landscape and its potential impact on environmental and human health.

Rubber Mulch

According to the EPA, the US discards 290 million tires each year. That’s a fair chunk of rubber. As these designated tire dumps fill up, disposal companies have found a second-life for scrap tires in the form of rubber mulch.

My first experience with rubber mulch came while I was a student at SIUC. The school had just started using rubber mulch in their parking lot planting islands. As horticulture students, we were attracted to this mulch purported to last ‘forever’. Plus the appearance was remarkably similar to shredded wood mulch and the rubber mulch was, of course, dyed our school colors, Saluki Maroon. What’s not to love?

The infatuation didn’t last long. As the weather warmed the rubber mulch baked amidst the hot parking lot giving off a pungent smell of hot tires. Additionally, the rubber mulch did not last forever. In fact, there are certain bacteria that feed readily on rubber material. Over time the painted coating faded and left behind readily identifiable shreds of used tires.

Dr. Linda Chalker-Scott, Extension Specialist from Washington State University, has spoken and published articles on the use of rubber in the landscape. In these presentations, she discusses the benefits and detriments of rubber mulch through various studies. Following are some key points from her work:

Impact on weeds – Rubber mulch is less effective in controlling weeds than organic mulches

Impact on Plants – Studies identified symptoms of leaf yellowing, reduced tree growth, and increased tree mortality. Plus, soil tests show high levels of zinc which can be toxic to plants and could cause leaf chlorosis.

Impact on pests – Asian cockroach prefers rubber mulch over others.

Flammability – When ignited rubber mulch is most difficult to extinguish.

Decomposition – Rubber mulch is not permanent. As the product decomposes breakdown products such as heavy metals, plasticizers, and accelerators are released into the environment.

Human Health Concerns – There is building evidence on toxins identified in crumb rubber. (The material used to cushion artificial turf) Crumb rubber is a near-identical material as rubber mulch, it is just much smaller. Studies have shown athletes playing on artificial turf with crumb rubber gain high exposure to these toxins through inhalation of the dust stirred up from play. There are, however, no similar studies on rubber mulch.

Convenience is rubber mulch’s primary benefit. Rubber mulch comes in a variety of colors to suit a homeowner’s plant palette and home décor. Along with being easy to apply, rubber mulch is also easy to find as it is sold in most garden centers. Additionally, rubber mulch’s decomposition rate is much slower than comparable mulches so reapplication is infrequent.

Rubber mulch is commonly used on playground surfaces and for good reason as this material works exceptionally well at cushioning impacts.

Finally, an interesting point brought up by Dr. Chalker-Scott is that the EPA classifies tires not dumped in an approved landfill as pollutants. So how does this differ from the legal “dumping” of shredded tires as landscape mulch?




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Chris Enroth is a horticulture educator with University of Illinois Extension, serving Henderson, McDonough, Knox, and Warren counties since 2012. Chris provides horticulture programming with an emphasis on the home gardener, landscape maintenance personnel, and commercial landscapers. Additional responsibilities include coordinating local county Master Gardener and Master Naturalist volunteers - providing their training, continuing education, advanced training, seasonal events, and organizing community outreach programs for horticulture and conservation assistance/education. In his spare time, Chris enjoys the outdoors, lounging in the garden among the flowers (weeds to most).