Building an outdoor plant box requires selecting materials that are durable, weather-resistant, and aesthetically pleasing. The best options include cedar, redwood, and pressure-treated pine, which naturally resist rot and insects. Cedar is a top choice due to its longevity and resistance to moisture, while redwood offers a beautiful finish and similar durability. Pressure-treated pine is a budget-friendly alternative but requires proper sealing to prevent chemical leaching. For a modern look, consider composite materials or metal, though these may need additional insulation for plant roots. Always ensure proper drainage and avoid toxic treatments if growing edible plants. With the right materials, your outdoor plant box will thrive for years.