I recommend using SoyShield Premium stains and sealers. They are bio-based, nontoxic sealers made from soybeans. Due to the nontoxic nature of the formula, hazardous chemicals will not leach into the garden.
The sealer comes in several colors: clear, honey, cedar, redwood, and walnut. Coat all six sides of the wood for maximum protection. This will keep water out and help prevent rot and decay.
For more information: For further advice on building safe and durable garden planters, read Polly Osborne's Ask A Pro Q&A article "Can you suggest an alternative to pressure-treated wood for garden borders and raised planters?"