Proper drainage is the most critical factor for success with DIY urban planter boxes. Without adequate drainage, plants can develop root rot and other moisture-related issues. The best arrangement for drainage holes depends on your planter's size and material.
For standard rectangular planter boxes under 4 feet in length, space drainage holes approximately 6-8 inches apart in a grid pattern across the entire bottom surface. Use a drill bit between 1/4-inch and 1/2-inch in diameter - larger holes are better for preventing clogging. For planters longer than 4 feet, increase the number of rows of drainage holes to ensure even water drainage.
If building with wood, drill holes completely through the bottom boards before assembly for cleaner results. For plastic or metal containers, use a sharp drill bit appropriate for the material. Never create just one large central drainage hole, as this can create a single point of failure. Multiple smaller holes distributed across the bottom provide more reliable drainage.
Place a layer of landscape fabric or mesh over the holes before adding soil to prevent soil washout while maintaining water flow. Elevate your planter slightly using feet or spacers to ensure drainage holes remain unobstructed. This simple arrangement strategy will keep your urban garden thriving throughout the growing season.