I built these with treated wood for the base and all cedar wood for the box. Your soil will not contact the treated wood.
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These are built to last! I drilled close to sixty holes for pocket screws and used plenty of outdoor wood glue when assembling them. They easily support my 240 lbs. They are sanded smooth and treated with outdoor oil based stain (green) or boiled linseed oil.
These are about 4 feet long, almost 2 feet wide and the top is 30 inches from the ground. Please contact me with any questions. Thanks for looking!