Facts about rubber mulching

Manufacturers and distributors claim recycled rubber mulch is an environmentally friendly, non-toxic choice for landscapes. It's true that discarded rubber tires are a huge waste issue because, not only are they slow to decompose, they are also a significant fire hazard. According to the Environmental Protection Agency, we generate 290 million scrap tires each year. Alternative uses for this waste, such as rubberized asphalt, playground surfaces or landscape mulches, seem like the perfect solution.

When I want to know the research on horticulture topics, I turn to Linda Chalker-Scott, horticulturist and Extension professor at Washington State University. In reporting research on rubber mulch, she found that these products "were less effective in controlling weeds in herbaceous perennial plots than wood chips." Another study she looked at "found rubber to be less effective than straw or fiber mulch in establishing turf grasses."

Rubber mulch ignites more easily and burns hotter with higher flame lengths than pine needles or bark mulches of various types and sizes, according to research by University of Nevada Cooperative Extension Natural Resource Specialist Ed Smith in cooperation with Carson City Fire Department.

Although manufacturers claim rubber mulch is permanent, certain bacteria do break it down. Once the rubber is broken down, leaching toxic substances into soil and water can become an issue.

"If rubber products have been exposed to contaminants during their useful lifetime, such as lead or other heavy metals, they will adsorb these metals and release them as well." (Chalker-Scott). Zinc can make up 2 percent of tire mass, and when leached into the soil, it can cause zinc toxicity to plants. Some rubber leachates can also harm humans. Effects of exposure range from skin and eye irritation to organ damage, nerve damage and possible death.

Finally, since some tires are steel-belted, small pieces of steel can sometimes be found in rubber mulches, making it unsuitable for playgrounds or play areas.

The bottom line per Dr. Chalker-Scott:

n Rubber mulch is not as effective as other organic mulch choices in controlling weeds.

n Rubber mulch is highly flammable and difficult to extinguish once it is burning,

n Rubber mulch is not permanent; like other organic substances, it decomposes.

n Rubber mulch is not nontoxic; it contains a number of metal and organic contaminants with known environmental and/or human health effects.

More information at theinformedgardener.com

JoAnne Skelly is the Carson City/Storey County Extension educator for University of Nevada Cooperative Extension and may be reached at skellyj@unce.unr.edu or 887-2252.