Choosing the correct depth for your urban planter boxes is crucial for the health and longevity of small shrubs and ornamental grasses. These plants require adequate soil volume for their root systems to establish and access nutrients and water.
For most small shrubs, such as dwarf hydrangeas, boxwoods, or hebes, a planter box depth of 45 to 60 centimeters (18 to 24 inches) is generally recommended. This depth allows room for root growth and provides a stable base for the plant.
Ornamental grasses, like blue fescue, fountain grass, or Japanese forest grass, can often thrive in slightly shallower conditions. A depth of 30 to 45 centimeters (12 to 18 inches) is typically sufficient for many varieties. However, taller or more vigorous grasses will benefit from being in the deeper range, closer to 45 cm.
Beyond depth, ensure your planter has excellent drainage holes. Use a high-quality, well-draining potting mix, not dense garden soil. Consider the mature size of your plant—both in height and root spread—when selecting the planter's width as well. A container that is too shallow will restrict roots, leading to stunted growth, increased water stress, and potential tipping. By providing the right foundation, your container garden will flourish for seasons to come.