[No. 04] Gardner is sour on rubber mulch. SynTurf.org. Newton, Mass. 3 May 2012. A few days ago, we learned about a gardener whose experience with rubber mulch did not turn out to be a big hit. This is what she wrote to Enviornment and Human Health, Inc. (www.ehhi.org) :
I am an avid gardener, and was very excited a year ago to find recycled rubber tires being sold as mulch! How smart and environmentally friendly, I thought, to find a use for old rubber tires! I bought two big bags of it.
YIKES - this year, for the first time since we bought our home in the year 2000, our beautiful weeping cherry tree has not budded out at all, and it seems to be dead!
Also, ZERO “perennial” herbs have come up in my outdoor garden that I mulched with that stuff last year, even though we had a VERY mild winter and a warm spring and my wonderful oregano and rosemary are ALWAYS peeping up by now, even after really hard winters.
One other weird thing: my very pretty “ice plants,” a type of small perennial with various brilliantly colored blooms that have been in my rock garden for years, all came up, but with the same, sickly “straw” color! I fear I killed everything by using that mulch.
I scratched up as much as I could as put it in a garbage bag (so now it will go to the landfill: YIKES) but a lot of it sunk into the ground in my gardens and around my trees.
If this story is a familiar one, then perhaps you may wish to consult with an agronomist to determine exactly what caused the degradation to your garden.
This story has provided us with an opportunity to update what we know about the lethal nature of excessive zinc in rubber mulch, which is capable of killing or degrading vegetation.
The potential problem of zinc leaching into the environment from crumb rubber in artificial turf fields prompted us to devote on this website a separate page on the topic of zinc (http://www.synturf.org/zinc.html). There, we reported on the following:
In an effort to find materials to supplement pine bark media, some growers have considered using ground rubber from recycled tires. Ground rubber is free of most disease pathogens, provides good drainage, resists decomposition and is both inexpensive and widely available. However, it has one critical attribute that may negate all of these seemingly good qualities: high levels of zinc.
Tire manufacturing involves addition of zinc oxide to strengthen the rubber. Ground rubber contains 1.55 percent zinc. Under acid conditions in the soil, zinc bound in the rubber becomes available for plant uptake. Research shows that as little as 2 percent ground rubber mixed with sand causes a decline in plant growth that is directly attributable to zinc toxicity (Schulz 1987). Symptoms of this toxicity in amended media include wilting, discolored leaves and very high concentrations of zinc in the leaf tissue (Bowman and others 1994).
Although zinc is essential for plant growth, most landscape soils and media used for container-grown nursery stock already contain adequate levels. Therefore, addition of zinc from any source may cause more problems than it resolves. Ground rubber, as either a mulch or a media amendment, increases the potential of zinc toxicity, especially when coupled with application of micronutrients in the fertilizer. As the old adage says -
“Enough of a good thing is as good as a feast Š too much of a good thing is good for nothing….” - Theodore Hook.
As a rule, evaluate any potential soil or media amendment thoroughly before using it on a routine basis. Research on the effects of ground rubber on plants indicates that it is unsuitable for use in production of nursery crops (Handreck 1996).
- Bowman DC, Evans RY, Dodge LL. 1994. Growth of chrysanthemum with ground automobile tires used as a container soil amendment. HortScience 29:774-6.
- Handreck A. 1996. Zinc toxicity from tire rubber in soilless potting media. Commun Soil Sci Plant Anal 27(13 & 14):2615-23.
- Schulz M. 1987. Effects of ground rubber on Phaseolus vulgaris. Z Pflanzrnahr Bodenk 150:37-41.
REACH stands for the Registration, Evaluation and Authorization of Chemicals. REACH involves thousands of chemicals and materials that are sold in the European Union. The purpose of REACH is to increase knowledge about chemical substances marketed in the EU, and their uses, and to ensure that they are properly managed. The REACH regulations entered into force in June 2007, with specific regulatory components entering into force in June 2008.
Under REACH, the heavy metals that are of concern include mercury, zinc, copper, cadmium, vanadium and lead: they are harmful if spread in the environment. Zinc is used as a curing activator for rubber and therefore its presence in crumb rubber in infill artificial turf fields is particular concern to environmentalists. Copper is used in pigments for plastics and rubbers. Cadmium is a cumulatively toxic element. And lead accumulates in biological systems and is linked to behavioral changes, paralysis and blindness. It was used as curing activator or stabilizer for certain polymers.
With specific focus on zinc, the publication points out that –
In 1993 the Dutch National Institute of Public Health and Environmental Protection (RIVM) presented an 'Integrated Criteria Document' for zinc, recommending for water a 'desirable' level of 9 µ/l and a maximum allowable concentration of 25 µ/l.
In 1995, zinc and zinc derivatives were included in a priority list of rubber chemicals compiled by the Swedish Environmental Protection Agency, which should be replaced or used restrictively.
In 1995, zinc and zinc oxide were placed on the second European list of priority substances in the EU Risk Assessment Programme.
In June 2002 the German Standard DIN 18035-7 "Sports Grounds, Part 7" "Artificial Turf Areas" was published. According to this standard, two leaching tests are required for post-consumer tyre rubber granulates used as infill material for artificial turf and the following limits are set in leachates:
•0.5 mg/l after leaching with deionized water (DIN 38414-4)
•3 mg/l after leaching with water saturated with C02
Between 1998 and 2004, draft Assessments were produced and responses put forward by the zinc chemical and rubber industries.
Since 29 April 2004 (see Council Directive 2004/73/EC, relating to the classification, packaging and labelling of dangerous substances) zinc oxide is officially classified as "Dangerous for the Environment" with the risk phrase "Very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment", and with the safety advice "This material and its container must be disposed of as hazardous waste" and "Avoid release to the environment. Refer to special safety instructions/safety data sheets". Rubber compounds containing more than 2,5 % in total of zinc chemicals or other chemicals classified as R50/63 (such as IPPD) are classified as "Toxic to aquatic organisms, may cause long-term effects in the aquatic environment".
[No. 02] Harm of zinc.The following is an excerpt from Chiara Canzi, “Turf v. grass: Have county schools rushed to judgment on the safety of synthetic turf?,” in Charlottesville News and Arts, January 13, 2009, available at
Although lead seems to have attracted the most attention, some believe another harmful chemical is found in the rubber. “The ground-up rubber tires have just a ton of zinc,” says Nancy Alderman, president of Environment and Human, Health Inc. Alderman’s organization is a Connecticut-based nonprofit that has questioned the safety of these synthetic turf fields for years. A study by the University of North Carolina at Chapel Hill cites cardiovascular damage as a possible consequence of continued exposure to zinc. A study by the North Carolina Department of Agriculture and Consumer Services linked high levels of zinc to a decline in plant growth. With as little as 2 percent of zinc mixed with sand, plants stopped growing as a direct cause of zinc’s toxicity. In addition to zinc, the rubber could contain other metals such as arsenic, cadmium, chromium, lead or selenium, according to Alderman.
[No. 01] In 2007, the National Institute for Public Health and the Environment (RIVM) in Bilthoven, Netherlands(www.rivm.nl) issued a study that examined the health and enviormental impact of zinc from artitfcial turf fields. Conducted for the benefit of the Dutch Ministry of Housing, Spatial Planning and the Environment, the study focused on on the release of zinc from the rubber infill of soccer fields, i.e. the zinc load, the distribution of zinc between the soil, roundwater and surface water. The study showed that zinc from the rubber infill is either emitted mainly to the surface water (when a drainage system has been constructed on clay or peat soils) or mainly to groundwater (in naturally well-drained sandy soils). The study also showed that the predicted concentrations of zinc in soil, under typical Dutch drainage conditions, also exceeds environmental quality standards. “The predicted zinc load is relatively high,” stated the study, exceeding even the zinc criteria in the Dutch Building Materials Decree. The leaching rate of zinc from rubber crumbs, the study found, was up to 20 times greater than the local leaching of zinc from agricultural applications of manure and pesticides. See A.J. Verschoor, “Leaching” of zinc from rubber infill on artificial turf (football pitches),” Bilthoven, Netherlands: National Institute for Public Health and the Environment (RIVM), RIVM Report No. 601774001/2007, March 12, 2007, available via http://www.rivm.nl/bibliotheek/rapporten/601774001.html, or directly at http://www.rivm.nl/bibliotheek/rapporten/601774001.pdf.