Choosing the correct depth for an urban planter box is crucial for the health of a small shrub. As a general rule, the planter should be at least 1.5 to 2 times the depth of the shrub's root ball. For most small shrubs, this translates to a minimum depth of 12 to 18 inches (30 to 45 cm). This provides ample space for root development, which is essential for stability, nutrient uptake, and overall plant vigor.
Several key factors influence this requirement. First, consider the shrub's mature root size. Research your specific variety. Second, excellent drainage is non-negotiable; ensure your planter has adequate holes and use a high-quality, well-draining potting mix. Deeper boxes also provide a larger soil volume, which retains moisture better and insulates roots from temperature swings common in urban settings.
A planter that is too shallow will restrict roots, leading to stunted growth, water stress, and increased susceptibility to wind. Always err on the side of a deeper container. By providing the right foundation—sufficient depth, proper drainage, and good soil—your small shrub will have the best chance to thrive in its urban container environment.