Experts and BHG readers answer.
Boxed in vegetable garden
"Raised beds of any type should be constructed out of rot resistant materials. Pressure treated lumber is usually the least expensive route to go, but some have concerns about the treated products leaching into the soil. You could line the wall of the bed with heavy plastic if this is a concern for you.
Other rot-resistant woods to consider are cedar and redwood. Costs of these vary from region to region, so you may want to check prices of what is available in your area. Other materials to consider are composite lumbers made from a combination of vinyl and wood fibers. These usually are more expensive, but will last longer than wood. In our own Test Garden, the cedar wood raised beds are rotting away after 10 years, but the ones built of composite materials show no signs of rot or decay.
If you make the raised beds deep enough, it isn't necessary to till up the underlying clay. However, mixing in some of the new soil with the existing clay can help prevent the formation of a soil texture barrier through which roots and water won't easily penetrate. The rooting depth for most vegetables should be at least a foot."