Based on the conflicting information, and the accusations that
studies showing no harm are conducted by rubber mulch companies
themselves, it appears you will have to decide on which side you want
to err. If you want to be "sure" that your plants will be healthy, and
that your soil will be free of "potential" leeching from rubber, then
you should go with bark mulch, gravel, etc. After all, we have only
had the option of recycled rubber mulch for a few years. We all did
fine without it until that time!
On the other hand, if you feel that the scientific studies do not
offer any substantial research regarding potential harm, and you want
to do some good for the environment by saving landfill space from
discarded tires, then that is a good choice as well!
I have chosen to exclude articles from the rubber mulch companies,
themselves, since they are certain to highlight the safety of their
product (which may, in fact, be quite safe). I simply decided to
include more objective sources so that you would be able to feel that
you have been presented both sides in a more unbiased manner.
Aside from the article noted by my colleage in the clarification
above, please see the following:
From "Part 4: How Mulch Work Do You Want To Do In Your Garden?," by
Teresa Watkins. July 7, 2004
"Rubber mulch is the latest innovation in mulches on the gardening
scene, again, with its pros and cons. The value of rubber mulch is
that it?s a pervious surface, allowing water to drain into the earth,
replenishing ground and surface water, and it?s not attractive to
insects. Rubber mulch is also an excellent groundcover for parks,
children?s playgrounds, parking lots, and personal car and boat common
areas. It is a softer surface so if you are concerned about children
hurting themselves, this is an excellent choice but make sure you buy
the better quality rubber and not the cheap rubber mulch which still
has protruding metal in it. With rubber mulch, you want quality. It?s
also a permanent alternative, not needing replacement, so make sure
it?s what you ultimately want. Rubber mulch has the distinct
advantages of having hundreds of rainbow choices to select with blues,
purples, greens, pinks, yellows, and oranges."
"I have seen a homeowner?s landscape mulched with teal blue rubber
around his tropical plants; it not only was colorful but also gave me
a feeling of Caribbean ocean coolness that you would not get with
wooded mulches. If you have a pool landscaped terrace, tropical
colored mulch would add to the festive surrounding.
* The disadvantages of rubber mulch are few, but there are some.
Mostly it?s the aestethics of using rubber - you may not want to use
artificial mulch in your organic landscape. Costs are also higher than
organic mulches but then you don?t have to pay to replace it every
** The other point is that the color may affect the temperature of the
rubber. Dark colors will absorb heat more readily than lighter shades.
As with dyed organic mulches, be careful using colored mulches in your
design as it may camouflage your flowers rather than highlight them.
*** Serious concerns regarding rubber mulch that have not been
academically substantiated fully is that of petroleum leakage and the
mulch's combustibility. There are environmental studies showing both
pros and cons of rubber mulch and individually each homeowner should
decide what is best for their landscape.
"If forest fires are a concern in your area and you want to use mulch,
you may want to choose the permanent solution of rocks, not wood or
rubber mulch around your house."
From "Is Rubber Mulch a Safe Option For Backyard Landscaping?" by June Fletcher
"From at least one environmental standpoint, you've got to love rubber
mulch --no trees die in its creation. Made from some of the more than
290 million tires discarded each year, the shredded stuff saves
landfill space, too. One manufacturer says that 80 scrapped tires are
used to create one cubic yard of shredded mulch."
"Though it hardly looks real up close -- indeed, many manufacturers
play up its fauxness by dying it bright red, teal or other bold colors
-- and it costs more than twice as much as wood mulches, it does have
good qualities. Unlike wood chips or bark, it doesn't wash away in
rainstorms, attract termites or carpenter ants, or rot. Its inherent
bounciness has proved especially useful in playgrounds; the U.S.
Consumer Product Safety Commission gave it the highest shock
absorption ratings of any kind of common playground covering,
surpassing wood chips, gravel and sand. The U.S. Special Operations
Command in Virginia Beach, Va., said it plans to use rubber mulch as a
ground covering under a military obstacle course."
"But the mulch is controversial, with many people worried, as you are,
about the material's possible harmful effects. I've seen several
studies that have declared rubber mulch to be non-leaching,
non-flammable and non-toxic to both plants and animals, but all were
sponsored by the companies that process the mulch. A Washington State
University study indicates that rubber can leach chemicals that
contaminate water and hurt marine life. Rufus L. Chaney, an
environmental chemist at the U.S.D.A. Agricultural Research Service,
says that his research shows that small amounts of zinc in rubber will
leach into soil over time. If your soil is alkaline and starved for
zinc, as many soils are in Western states, that could be a good thing.
But if your soil is acidic and has an adequate amount of zinc, as is
typical in the East, the zinc released by the tires could cause a
chemical overload that kills shallow-rooted flowers, shrubs and
vegetables. "Save rubber mulch for paths and playgrounds," he says."
"Brands also differ. Steel-belted radial tires are used in many mixes,
and the magnets employed to remove them don't always get all the tiny
wires out. Though processors say the steel rusts in about six months,
in the meantime, you could get nasty pricks from the chopped up wires
while you're spreading the mulch around. So buy mixes that are at
least 98% steel-free. Also, check out the warranty for any coatings
used on the mulch. Black tires can oxidize over time, giving them a
chalky surface; look for a mulch that has a UV-protective coating."
This is probably the best article in terms of pros and cons, while
containing some references to scientific studies:
"The Myth of Rubberized Landscapes - "Recycled rubber mulch is an
environmentally friendly, non-toxic choice for landscapes," by Linda
Chalker-Scott, Ph.D., Extension Horticulturist and Associate
Professor, Puyallup Research and Extension Center, Washington State
From the conclusion:
The Bottom Line
? Rubber mulch is not as effective as other organic mulch choices in
? Rubber mulch is highly flammable and difficult to extinguish once it is
? Rubber mulch is not permanent; like other organic substances, it decomposes
? Rubber mulch is not non-toxic; it contains a number of metal and organic
contaminants with known environmental and/or human health effects
From "Landscaping with Mulch."
"Inorganic Mulches are man-made. They work well and seldom need
replacing, but they offer nothing back to the soil."
"Recycled Rubber is a great landscape alternative to bark or stone.
Made from 100% recycled rubber product, It is safe long-lasting and
natural-looking. Rubber mulch does the things you want mulch to do:
helps retain moisture, moderates soil temperature, controls weeds and
looks good in your landscape.
Available in various natural shades to provide the look of wood bark,
rubber mulch will not fade. It is heavier than bark mulch and less
likely to blow away or float away during a heavy rain. Rubber mulch
only needs to be applied to half the depth of traditional wood mulch.
This product is excellent for playgrounds and is approved by the
American Society for Testing and Materials (ASTM) when maintained at a
6" depth (compared to a minimum depth of 9" for wood mulches)."
Read one reply from a short (older - 1997) message thread on the the
Sustainable Agriculture Network Web Site:
"As you can see below, Pat Millner sent me a copy of the questions and
responses about tires. I have been part of the USDA composting team
since 1973, and studied metals in soils and food-chain from any
source. Tires normally have 0.5-2% Zn. Most rubber items have this
much Zn. In many situations, when rubber was used in plant growth
media, or burned tires residues were on soils, Zn killed plants. There
is an interaction
between soil or medium pH and Zn toxicity. At a reasonable rate of
application, rubber would be a high grade Zn fertilizer over time
because the Zn in rubber is purified, with very low Cd
"I have cited a number of papers over the years about Zn phytotoxicity
from rubber, but not conducted research on this myself. This Spring, I
attended a meeting in NC where one speaker noted that farmers had
heard from one another that if you put a tire around a tree stump and
started it afire, it aided in buring the stump to the ground. But when
they tried to grow Zn sensitive crops, even at neutral pH, Zn killed
peanuts and some other species."
"Because of the numerous adverse effects of rubber-Zn, I have advised
against using rubber in any composting, or in any potting medium, and
casual dispersal of rubber on agricultural or garden soils. If you
need references, please write back. This topic was first reported in
the early 1970s, and then re-done in the 1990s by people who didn't
search the literature. A nice demonstration of negative effects of
rubber in potting media was recently reported from Australia by
Handreck, I think in Commun. Soil Sci. Plant Analysis."
Here is an abstract of the study referenced above:
"Zinc toxicity from tire rubber in soilless potting media." Handreck, KA
Communications in Soil Science and Plant Analysis [COMMUN. SOIL SCI.
PLANT ANAL.]. Vol. 27, no. 13-14, pp. 2615-2623. 1996.
"Petunia "Stereo Red" and Impatiens "Impulse Violet" were grown in
soilless potting media that contained either 0, 5, 10, or 20% by
volume of ground tire rubber. The pH of half of each medium was
adjusted to 5.0 and that of the other half to 6.5. Shoot weights
declined with increasing rubber content, due to increasingly severe
zinc (Zn) toxicity. Lower shoot weights at pH 6.5 than at pH 5.0 were
attributed to manganese (Mn) deficiency that was unrelated to the
presence of rubber. Extrapolation from the experimental data indicated
that Petunia "Stereo Red" would have been damaged by as little as a 2%
inclusion of tire rubber. The high risk of Zn toxicity means that
ground tire rubber should not be included in soilless potting media."
Read advice (scroll down) from "Using Shredded Tires as Garden or
Landscape Mulch," by Deborah Brown, Extension Horticulturist. Yard and
Garden Line News. 2004.
Not scientific, but and interesting compilations of opinions from the
following message board thread: (I especially like the last post!)
The following studies involve rubber mulch products, but have no real
relevance to application in home gardening beds. However, you might be
"FOLIAR ACCUMULATION OF ZINC IN TREE SPECIES GROWN IN PINE BARK MEDIA
AMENDED WITH CRUMB RUBBER." Edward Bush A1, Kris Leader A1, Allen
Owings A1. Journal of Plant Nutrition. Volume 24, Number 3 / 2001
"Incorporated crumb rubber (CR) increased Zn tissue levels up to nine
times the normal range in tree species. There was a linear increase in
Zn tissue accumulation with increasing percentages of crumb rubber for
river birch (Betula nigra L.), lacebark elm (Ulmus parvifolia Jacq.),
and pecan [Carya illinoinensis (Wangenh.) C. Koch]. Pecan leaves
containing high levels of Zn exhibited no visual symptoms, unlike the
remaining tree species exhibiting leaf chlorosis. Results suggest that
crumb rubber incorporated at rates greater than 25% may cause
abnormally high concentrations of Zn to accumulate in plant tissue."
"Adaptation of Waste Tire Rubber for Greenhouse Media and Zinc
Fertilizers." Submitted to: Colorado Advanced Materials Institute.
Colorado School of Mines.
"Evaluation of Recycled Rubber Mulch Products," by David M. Jared.
Research and Development Branch. Office of Materials and Research.
Georgia Department of Transportation. November 2001.
"Soil nutrients and microbial biomass following weed-control
treatments in a Christmas tree plantation," by MA Arthur and Y Wang.
Soil Science Society of America Journal, Vol 63, Issue 3 629-637,
Copyright © 1999 by Soil Science Society of America.
As you can see, there is no extensive scientific research on this
topic. I think I would be far more comfortable trying rubber mulch in
a landscape area than in a food garden! Maybe you should try your own
experiment. Try rubber mulch in one area, and bark mulch (or normal
old dirt!) in another, and take some soil tests of your own (or have
your county extension agent get involved.)
It would be interesting to see what you find out.
I hope this information is helpful!
is rubber mulch harmful?
pros and cons of rubber mulch
research studies on harmful effects of rubber mulch
rubber in potting soil