When incorporating small shrubs into urban gardens or balcony spaces, selecting a planter box with adequate depth is crucial for plant health and longevity. Generally, for most small shrubs like dwarf hydrangeas, compact roses, or boxwoods, a minimum depth of 12 to 18 inches (30 to 45 cm) is recommended. This provides sufficient room for the root system to establish and access necessary nutrients and water.
The required depth depends on several factors. First, consider the shrub's mature root size. Research your specific shrub variety to understand its root spread. Second, ensure excellent drainage. Deep planters prevent waterlogging, but they must have drainage holes. A layer of gravel or broken pottery at the bottom can further aid drainage. Third, use a high-quality, well-draining potting mix formulated for shrubs or trees, not dense garden soil.
For evergreen shrubs or those in windy, exposed locations, a deeper and heavier planter (around 18-24 inches) adds stability. Remember, deeper soil also retains moisture longer, reducing watering frequency. Ultimately, providing ample root space is the key to helping your small shrubs thrive in an urban planter box, bringing lasting greenery to your city space.