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Green Speak

Contaminated Landscape Materials: Rubber Mulch

Despite our best intentions to create healthy gardens and landscapes, sometimes we wind up introducing a material that has 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 on rubber material. Over time the painted coating faded and left behind easily identifiable shreds of used tires.

Dr. Linda Chalker-Scott, Extension Specialist from Washington State University, recently gave a webinar on the use of rubber in the landscape. During her presentation she discussed the benefits and detriments of rubber mulch discovered through various studies. Following are some key points from her presentation:

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 causing chlorosis.

Impact on pests – Asian cockroaches 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 question brought up during the presentation is that the EPA classifies tires not dumped in an approved landfill as pollutants. So how does this differ from the legal “dumping” as landscape mulch?

Dr. Linda Chalker-Scott's presentation was recorded and can be viewed HERE. She also gives some great alternatives to rubber mulch.