Choosing the correct depth for your urban planter boxes is crucial for the health and longevity of small shrubs. While specific needs vary by species, a general rule is to provide a depth of at least 18 to 24 inches (45 to 60 cm). This depth accommodates most small shrub root systems, allowing for proper development, stability, and access to nutrients and water.
Key factors to consider include the shrub's mature root size—always research your specific plant. Excellent drainage is non-negotiable; ensure your planter has ample drainage holes and consider a layer of gravel or broken pottery at the bottom. Use a high-quality, well-draining potting mix designed for containers, not dense garden soil. Deeper planters also provide better insulation for roots against temperature extremes common in cities.
Ultimately, when in doubt, opt for a deeper planter. It offers more soil volume, which retains moisture longer and gives roots more space to grow, leading to a healthier, more resilient shrub in your urban garden.