The Tire Garden
Making a garden bed by stacking two or three used tires is another option for a small-space garden in which to grow potatoes, tomatoes, melons, or cucumbers. The soil in black-tire beds will become very hot, too warm for most plants, especially if the beds are placed in a sunny location. Choosing to grow heat-loving plants is the best option for this type of bed. If the area is shady or you live in a colder climate, the heat absorbed by the tire garden can be beneficial even for other veggies.
There is some debate as to whether or not used tires are safe for growing vegetables. Since synthetics are used to manufacture tires, when the rubber slowly degrades, it releases zinc and carcinogenic PAHs (polycyclic aromatic hydrocarbons) and other compounds into your soil. The amounts released are extremely small, and happen over a long period of time. This action may be a concern if you want to grow organic veggies. There is no evidence that these by products are harmful to the plants or the soil; however, some individuals are very concerned about any release of potentially harmful substances when growing food. Some gardeners suggest using the tire bed for growing flowers or shrubs rather than edible plants.