Choosing the correct depth for your urban planter boxes is crucial for the health of small shrubs. While specific needs vary by species, a general rule is to provide a planter depth of at least 12 to 18 inches (30 to 45 cm). This depth accommodates most small shrub root systems, allowing for proper growth and stability.
Key factors to consider include the shrub's mature root size, which you can research before planting. Excellent drainage is non-negotiable; ensure your planter has adequate holes and consider a layer of gravel or broken pottery at the bottom. Using a high-quality, well-draining potting mix, rather than garden soil, is essential for container health. Deeper planters also provide better insulation for roots against temperature swings common in urban settings. For marginally hardy shrubs or in very hot climates, opting for the deeper end of the range (18-24 inches) can offer a significant buffer. Remember, a deeper planter promotes healthier roots, which leads to a more resilient and flourishing shrub in your city garden.