Answer: Railroad ties are treated with creosote which isn't the best chemical to put in your yard. While the wood won't rot for decades, the substance can leach out into the soil so I wouldn't recommend using them to build planter boxes. Instead, use concrete blocks or bricks for a permanent structure, or untreated landscape timbers such as redwood or cedar which are naturally rot resistant and will last longer than softer woods. Good luck with your new planter boxes.
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