Starting an urban herb garden is one of the most rewarding projects for any city dweller. The fresh flavors right outside your window are unmatched. But a common and crucial question arises: how deep should my planter boxes be? The answer isn't one-size-fits-all, as different herbs have different root system needs.
For most popular culinary herbs, a planter box depth of 6 to 12 inches (15 to 30 cm) is sufficient. This range accommodates the root systems of staples like basil, cilantro, oregano, chives, and mint. However, understanding the specifics will set your garden up for long-term success.
Shallow Root Herbs (6-8 inches):
Herbs like chives, cilantro, and thyme have relatively shallow roots. A planter that is 6 to 8 inches deep provides them with ample space to establish themselves and thrive.
Medium Root Herbs (8-12 inches):
This is the sweet spot for the majority of common herbs. Basil, parsley, mint, oregano, and sage will all flourish in planters with a depth of 8 to 12 inches. This depth allows for robust root development, which supports a larger, more productive plant.
Deep Root Herbs (12+ inches):
Some herbs, like rosemary and lavender, develop deeper, more extensive root systems and can grow into small shrubs. For these, opt for a planter that is at least 12 inches deep to prevent them from becoming pot-bound and stressed.
Beyond depth, ensure your planter boxes have adequate drainage holes. Waterlogged soil is a primary cause of failure in container gardens. A layer of gravel at the bottom is a common suggestion, but modern horticultural wisdom suggests this can actually worsen drainage. Instead, simply use a high-quality, well-draining potting mix.
By matching your planter box depth to the herbs you wish to grow, you provide the foundation for a lush and bountiful urban garden. Happy planting