Ensuring proper drainage is the most critical step for success with urban planter boxes. Without it, plant roots can suffocate and rot. The best arrangement follows a simple, effective layered system.
Start by ensuring your planter has adequate drainage holes. Cover these holes with a permeable barrier, such as a piece of landscape fabric or a few broken pottery shards. This prevents soil from washing out while allowing water to flow freely.
The primary drainage layer should consist of coarse materials like horticultural gravel, pebbles, or clay pellets (LECA). This layer, typically 1-2 inches deep, creates an empty space for excess water to pool away from the soil above.
Next, add a separation layer. Place a sheet of landscape fabric or a fine mesh screen over the gravel. This crucial step prevents the potting soil from gradually settling down into the drainage layer and clogging it over time.
Finally, fill the remainder of the box with a high-quality, well-draining potting mix. Avoid using garden soil, as it compacts easily. Remember, the goal is to create a system where water can exit quickly, leaving the soil moist but not waterlogged, which promotes strong and healthy root growth for your urban garden.