The debate is still on as to the danger of using pressure treated wood in the garden. While little leaches out of intact timbers in a garden, some gardeners prefer to avoid it. The same is true for old railroad ties. Non treated woods will decay so fast that they are not worth the time and money. There are other woods that work well and resist decay, but may be very expensive. These include cedar, redwood and cypress.
Another option if you want to use treated wood, is to line the bed walls with a thick plastic sheeting to prevent the roots and soil from contacting the treated wood.
Other materials include cinder blocks, decorative stone, and preformed retaining wall stones sold by home do-it-yourself centers. Then again you could just use a non-bordered raised bed and count on doing some occasional work to keep them maintained as erosion wears them down.
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