Several gardening trends are colliding. Lots of new gardeners want to grow vegetables and raised beds are very popular. Combine that with a desire to be green and reuse material and tire gardens seem to be a good option. These are gardens made with old discarded tires.
Tires have another big advantage. They accumulate heat which potatoes and tomatoes just love, especially in colder climates.
But are these tires safe? Do they leach toxic material that will get into the food and contaminate it? There are lots of opinions on this, but very little in the way of a scientific review. Time to change that.
Why Use Tires
Used tires are easy to get for free, and you don’t have to be handy at construction to build a raised bed. Plop them on the ground and you are done. You can then fill them with soil in a more traditional raised bed or you just plant in the soil under them in which case the tire radiates heat to the plant and keeps it warmer.
No cost, no rotten wood, no preservatives – it can’t get much easier than this.
Are Tires Toxic?
Tires contain all kinds of toxic organic chemicals. They also contain a number of heavy metals including cadmium, lead, and zinc. But that is really not the question that needs to be asked. The important question is, do these toxic substances leach out of the tire? Keep in mind that tires are designed to take a lot of wear on the road. They don’t exactly fall apart sitting in the sun. If they did, we couldn’t drive on them.
How stable are the tires? How much toxic material actually leaches into the soil in any given year? If this amount is very small, then they really are not toxic to gardening. It’s all about the dose.
There is a second important question that should also be asked. Will plants absorb these released toxins? Tires might seep some toxic material into soil, but if the plants don’t absorb it, then the toxins do no harm to the plants or to us as we eat them.
Do Toxins Leach Out of Tires?
It seems as if nobody has ever tested the movement of toxins from tires to plants. Some studies have looked at the movement of tire toxins into the environment while we drive, but that includes a lot of friction and higher temperatures, so you can’t really extrapolate that data.
Tire gardens use old tires, so much of the surface chemicals that might leach off them has already done that.
We do know rubber is quite stable. Tires sit in the environment for many years without showing signs of decomposition which means that little if anything is coming out of them.
One use for tires is to break them up into small pieces, called tire crumb, and use it for paving and children’s playgrounds. Studies on this use found that, “the use of tire crumb in playgrounds results in minimal hazard to children and the environment. Toxicity to all aquatic organisms (bacteria, invertebrates, fish, and green algae) was observed; however, this activity disappeared after three months of use”.
Creating crumb exposes internal surfaces that have not been weathered so it is no surprise that there is leaching from new tire crumb. What is surprising to me is that it mostly stops after 3 months. We can expect that used tires will show even lower toxicity than three month old tire crumb.
A study looked at top soil contaminated with new tire crumb and measured effects on soil life. Microbe activity was unaffected, earthworm survivorship was not impacted, and “earthworms living in contaminated soil gained 14% less body weight than did earthworms living in uncontaminated soil.” There was no measured increase in heavy metals except for zinc.
Although there is limited data available, tire gardens probably leach very little toxic material to the soil.
Tires Might Reduce Toxins?
A study looking at waste water showed that rubber tire crumbs removes naphthalene, toluene, and mercury from water. It is especially efficient at removing mercury.
Do Plants Absorb Toxins?
Plants can absorb some toxins and they are so good at this that they are being used as a way to remediate contaminated soil. Organic compounds may be degraded inside the plant into harmless molecules, however, heavy metals remain in the plant and could end up in the food.
Since rubber tires produce a lot of different chemicals, it is complicated to understand how each type of plant deals with each chemical. Lets have a look at zinc as an example.
Zinc is required by plants and plays an important role for several plant enzymes. Plants benefit from added zinc in soils that have low levels. As the pH increases, plants have a harder time absorbing zinc from the soil and are more likely to show zinc deficiencies.
The point here is that some zinc is good for plants and animals. Small additions of zinc from tires may actually be good for plant growth.
Are Tire Gardens Safe?
There is not enough scientific data to answer this question with any certainty. The best we can do is make an educated guess.
Except for zinc, heavy metals are probably not an issue since tires don’t leach them in significant amounts. There is no indication that zinc levels will reach toxic levels in soil or the plants.
Consider these facts:
- The growing time of vegetables in tires is relatively short.
- Leaching of chemicals from tires is a slow process.
- Microbes in the soil are able to degrade many organic pollutants.
- Vegetables from our own garden normally do not make up a major portion of our yearly diet.
Based on these facts and on what we do know, tire gardens are probably safe.
Would I use them? No. The slight benefit of heat does not justify the potential risk, besides I don’t like the smell of hot rubber. But then I also don’t use raised beds for growing tomatoes, so I have little use for tires.
The benefits of raised beds are over stated. They do offer some relief from bending in certain medical conditions, but growing on level soil works just as well for most gardeners. For a more thorough review of this, see: Raised Beds – Pros and Cons.