For urban gardeners, one of the most common questions is: how deep do my planter boxes need to be for a successful small herb garden? The answer is crucial for healthy plant growth. Generally, most popular culinary herbs require a planter box depth of 6 to 12 inches (15 to 30 cm). This range accommodates the root systems of herbs like basil, cilantro, oregano, thyme, and mint.
However, depth requirements can vary. Shallow-rooting herbs such as chives, thyme, and creeping oregano can thrive in containers as shallow as 6 inches. Medium-rooting herbs like basil, parsley, and cilantro prefer at least 8-10 inches of soil depth. For more robust or perennial herbs like rosemary, lavender, or mint (which is best grown alone due to its invasiveness), aim for a deeper planter of 12 inches or more to allow for extensive root development and better overwintering.
Beyond just depth, ensure your planter boxes have adequate drainage holes to prevent waterlogged soil, which is detrimental to herbs. Use a high-quality, well-draining potting mix rather than garden soil. For a mixed herb garden, a depth of 10-12 inches is a safe and versatile choice, providing enough room for a variety of roots to establish. Remember, a deeper soil volume also retains moisture better and provides more nutrients, reducing the frequency of watering and feeding. With the right depth and care, your compact urban planter can yield a bountiful and aromatic herb harvest all season long.