For thriving lettuce in your urban planter boxes, soil depth is crucial. Fortunately, lettuce has a relatively shallow root system. A depth of 6 to 8 inches (15 to 20 cm) is generally sufficient for most loose-leaf and butterhead varieties. This allows adequate space for root development and moisture retention.
While 6-8 inches is the sweet spot, you can grow smaller varieties like baby greens in containers as shallow as 4 inches. For larger heading types like romaine, opting for 8-12 inches provides extra room for robust growth. The key factor is not just depth but ensuring excellent drainage. Use a high-quality potting mix and ensure your planter has drainage holes to prevent waterlogged soil, which can cause root rot.
To maximize your harvest, choose a planter that is wider rather than deeper. This allows you to space plants appropriately for continuous "cut-and-come-again" harvesting. Remember, consistent watering is more critical than extreme depth, as shallow soil dries out faster. With the right soil depth and proper care, you can enjoy fresh, homegrown lettuce from your urban garden all season long.