When growing small shrubs in urban planter boxes, depth is a critical factor for their health and longevity. The general rule is to provide a planter box with a depth of at least 45 to 60 centimeters (18 to 24 inches). This depth accommodates the root ball of most small shrubs and allows adequate space for root development, which is essential for stability and nutrient uptake.
Several factors influence this requirement. First, consider the mature size of the shrub's root system. Dwarf or compact varieties are best suited for containers. Second, excellent drainage is non-negotiable; ensure your planter has sufficient drainage holes to prevent waterlogged soil, which causes root rot. Third, use a high-quality, well-draining potting mix designed for shrubs or trees, not dense garden soil.
For optimal results, select a planter that is both deep and wide enough. A depth of 50-60 cm (20-24 inches) is an excellent target for most small shrubs like boxwood, hydrangeas, or dwarf conifers. This depth supports the roots, insulates them from temperature swings, and provides a reservoir for moisture and nutrients. Always check the specific depth requirements for your chosen shrub variety, as some may need slightly deeper or shallower conditions to thrive in an urban container garden.