Woah there! Rubber tires are very toxic. Disposing of them is often treated as hazardous waste. The fact that little to nothing grows through them is definitely not a selling point, especially when considering their application in horticulture. Rubber dust (from tire wear) is considered one of the reasons that cities in general, and living close to any major traffic corridor is statistically more likely to give you cancer, as well as other diseases from weakened immune systems. They are nasty blends of substances hostile to life. Just think about walking into a tire store or mechanic's garage on a summer day, where tires are piled up. Remember the smell? Did you leave with a headache, an inexplicable weariness, or a bit queasy in your stomach; possibly even a bit melancholic for no specific reason? That's from breathing in the fumes emitted by the tires. The fumes lessen over time, as they get dumped into our air and water, but there will be plenty left in the shreds (and shredding massively increases surface area for fresh off-gassing). So tempting as they may be, best avoided. erik