Tire gardens have their uses. You can plant on concrete or above poor soils. You can raise them on supports to keep vegetables out of animals’ reach. If you sell at market, your produce is mobile; you can bring your crop to market and harvest only what sells.
Tires have been used for vermicompost bins and for herbs. They make a nice addition to a kitchen garden, being accessible right out the front door. Best of all, they are a cheap raised bed, and reuse an otherwise voluminous waste product.
We use them at the ECHO Asia farm for growing perennials and annuals. We have fruit trees planted in tires to get them started. We have tomatoes for seed production planted in tires. Tires are not used exclusively by ECHO, but have been promoted by ECHO as a container option for urban gardens.
At the ECHO Global Farm in Florida, the first question I heard as an Urban Garden intern was, “Is that produce safe to eat?” I did not know the answer. When Stacy Swartz gave a seminar on small-scale research, she told us to consider projects in terms of relevance to our network and the effort required. For the urban garden, the question of whether tire gardens were toxic came up regularly, and all I needed to do was send in a soil sample to a lab. So we did it. We compared soil and garlic chives that had spent 20 years in a tire with the soil and garlic chives planted directly in the ground in another section of the farm known as the Lowlands. These were our results:
Tire Garlic Chives: 1.17 ppm lead
Tire Soil: 9.88 ppm lead
Lowlands Garlic Chives: Below Detectable Levels
Lowlands Soil: 4.28 ppm lead
The World Health Organization (WHO) points out that there is no known, safe level of lead for small children (WHO 2018). According to international food standards, leafy vegetables should not have more than 0.3 ppm lead (FAO/WHO 2015). Twenty years of perennial garlic chives in the same tire is a worst case scenario, but if a perennial is planted in a tire, it is a lot of work to move them and tires last a long time. One could easily leave a plant for a decade or two.
The Environmental Protection Agency (EPA) notes that soil normally contains lead at less than 50 ppm (ATSDR 2017; Grubinger and Ross 2011). Lead concentration in our soil is well below 50 ppm, suggesting that the garlic chives have picked up the lead from some other source or for some other reason. We do not know whether the roots in contact with the tire wall or pooled leachate in the tire rim is the cause, or whether a lack of nutrients caused the roots to uptake heavy metals. Both may have contributed.
The question now is how long does it take for the heavy metal contaminants to build up to a dangerous level? We are beginning to test this at ECHO Asia by setting up an observational trial for 10 tires. Over the next 10 years, the research staff will test the soil every year for either lead, cadmium, or chromium on a rotating basis. If the soil is contaminated, we will test the garlic chives planted in the tires.
The experiment is designed to suggest the boundaries of risk and what should be studied more. We are comparing worst case scenarios–extremely degraded tires–with tires treated to prevent contamination–turning the tires inside out, lining them with plastic, or painting them. We also have an untreated tire to see a ‘normal’ situation and two plastic containers as controls.
We took baseline data for this experiment at ECHO Asia and found our garlic chives—recently transplanted from a raised bed into tires–are below 0.05 ppm, as close to the previously-mentioned maximum allowable limit of 0.3 ppm as we could measure. Our initial soil mix is at 20.33 ppm. This is double the level in the 20-year-old tire’s soil in Florida (9.88 ppm), but still within the EPA’s normal limit of 50 ppm.
My hope is that we can write a small update every year-or-two to share what we have found about tire safety: whether the lead levels increase or stay the same. From what I could find on the internet, no one has taken the time to study this question in depth, so even our initial results of the 20-year old garlic chives will add to the conversation.
Below is some information referenced above and for further reading. Let me know if you have any other thoughts, particularly insights on knowing the risks of gardening in tires and how to minimize those.
Information on thresholds of heavy metals in soil:
Ministry of the Environment, Finland, 2007. Government Decree on the Assessment of Soil Contamination and Remediation Needs
Agency for Toxic Substances and Disease Registry. 2017. Case Studies in Environmental Medicine (CSEM): Lead Toxicity
Grubinger, V. and D. Ross, 2011. Interpreting the Results of Soil Tests for Heavy Metals. University of Vermont Extension.
Fisher, B. and ECHO staff. 2016. Tire Contaminants from a Container Gardening Perspective. ECHO Development Notes 130:1-3
Information on thresholds of heavy metals in plant tissue:
FAO/WHO (2015). General standards for contaminants and toxins in food and feed (CODEX STAN 193-1995)
Information on lead poisoning and human health
WHO. 2018. Lead poisoning and health.