Tires

Recycled tires are cheap (or free) and highly available. We've seen them used in the construction of Earthships and other alternative building structures, and many folks even use them in the garden. But, is this safe?

Safety

I'm linking to a study from the University of Wisconsin-Madison which indicates that tires used properly in construction are not at all hazardous. Tires are NOT appropriate, however, when stacked/exposed to the environment. It is not recommended to use tires in the garden, as they will constantly off-gas and degrade, releasing carcinogenic materials into your food.

• Tires are Safe, which references multiple studies.
• Summary from Earthship Biotecture folks

Alternatives in the Garden

Fortunately, there is no real need to use tires in the garden, as Potato Towers (and similar uses) can equally be constructed using wood, or even wraps of chicken-wire, as often is seen for leaf piles and Compost.