Rubber Mulch
By: Sandy Feather ©2006
Penn State Cooperative Extension

Q: I have recently seen man-made mulches from rubber. Can you give me any information about these products? Is it a good type of mulch to use? It is supposedly made from tires that have had all of the metal and other harmful products removed from them.

A: Rubber landscape mulch is sold under a number of trade names. It is made from shredded tires. Rubber mulches are touted as longer lasting than shredded bark, less attractive to insects such as ants and termites and aesthetically pleasing. They are advertised as safe for children, pets and plants because all of the metal belts and other harmful products have been removed as the tires are processed into mulch. Since discarded tires are a major waste management problem, it certainly sounds like a win-win situation to recycle them into a long-lasting, pest-free mulch.
  
Crumb rubber has been used successfully for many years as a soil amendment to reduce soil compaction on high-traffic lawn areas such as parks, putting greens and sports fields. Its use as surface mulch is a more recent development, so there is not as much research available. A point I see with rubber mulch -- like river rock or other non-organic mulches -- is that it does not have the benefit of adding organic matter to the soil as it breaks down.
  
I did find research from South Carolina's Department of Transportation that praised rubber mulch as having "a very pleasing appearance and [looking] remarkably like traditional wood mulch." The department also found fewer problems with insects when rubber mulch was used around buildings.
  

This brown rubber mulch wasn't enough to stop these puffballs from developing from buried wood beneath it

  
Some negative findings include a strong rubber odor on hot, humid days, stray pieces of steel strands, and that some annual flowering plants in very sunny areas did not survive due to excessive heat from the rubber mulch. They did not notice any negative effect on trees or shrubs. The cost of the rubber mulch was "considerably higher than traditional hardwood mulch [and] would have to last four to five years to be cost-effective."

The rubber mulch was monitored for only three years, but evaluators felt it would last the four or five years needed to justify the extra cost.

An article from Washington State University by extension horticulturist and associate professor Linda Chalker-Scott expresses a number of concerns about rubber mulches. In comparison studies, she found that rubber mulch was less effective in controlling weeds in herbaceous perennial and Christmas tree plots than wood chips. Another concern is that rubber is more flammable than traditional mulching materials and difficult to extinguish if it does ignite. Ms. Chalker-Scott also notes research from Bucknell University that leachate from car tires is deadly to aquatic organisms such as algae, snails and fish. The leachate contains heavy metals (especially zinc, which can account for as much as 2 percent of tire mass), plasticizers and accelerators used during the vulcanization process. Compounds used as rubber softeners and fillers also show up in the leachate, and also have been shown to be hazardous to aquatic organisms.

The leachate also may contain contaminants that the tires absorbed during their life on the road. Ms. Chalker-Scott points out that "a number of plant species, including landscape materials, have been shown to accumulate abnormally high levels of zinc, sometimes to the point of death."
  

Common types of mulch

Mulching Rhododendrons

Photos of mulches

Shredded mulch production

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