Liners for Outdoor Wooden Planters

My boyfriend and I are making wooden planters this weekend for my patio to have a garden. I was wondering if you knew of the best plant friendly substance I could use to protect the wood. We’re looking for a liner or some type of substance to put between the dirt and the wood to protect the wood from rotting without harming the plants. Any ideas?
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    Sarah Try a coconut liner or a poly pro plastic liner!