Planting vegetables, particularly potatoes in stacks of tires is pretty popular these days, but there is a growing question whether tire gardens are safe. While there has been no direct research on whether it is safe to grow vegetables in tire gardens, there is research that shows that as tires get older they degrade. According to Linda Chalker-Scott, Associate Professor and Extension Urban Horticulturist, WSU Puyallup Research and Extension Center, we do know that "rubber tires will be a constant source of leached metals, PAHs, and other pollutants as the tires degrade. Whether or not it's enough to worry about from a human health perspective is unknown."
While it does seem like the jury is still out about the potential health threat of tire gardening, it's important to keep in mind that the soil under the tire garden may become contaminated with hard to remove toxins as the tires degrade.
Another downside to planting in tires is that the black rubber can get extremely hot when sitting in direct sunlight, which can fry your plants.
If you do choose to grow vegetables in tires it's probably best to line them with plastic.For more information on gardening safety check out; The Informed Gardener and The Garden Professors