When starting an urban garden, one of the most common questions is about the necessary depth for planter boxes. The good news is that many popular herbs and small vegetables don't require extremely deep soil, making them perfect for container living.
For most culinary herbs like basil, cilantro, chives, oregano, and thyme, a planter box depth of 6 to 8 inches (15 to 20 cm) is perfectly adequate. These herbs have relatively shallow root systems and will thrive in these conditions, provided they have sufficient drainage and sunlight.
Small vegetables and leafy greens have slightly greater needs. For lettuce, spinach, arugula, radishes, and green onions, aim for a depth of 8 to 12 inches (20 to 30 cm). This allows for proper root development for these fast-growing crops.
For more substantial small vegetables like dwarf tomatoes, peppers, bush beans, or carrots (especially shorter varieties), you will need the deepest planters. A depth of 12 to 18 inches (30 to 45 cm) is ideal to support their root structures and ensure they have enough soil volume for nutrients and water retention.
Regardless of depth, two non-negotiable factors are drainage and soil quality. Every planter must have holes in the bottom to prevent waterlogging. Fill your box with a high-quality potting mix, not garden soil, as it is formulated to be well-aerated and drain properly in a confined space. With the right depth and care, your urban planter box can yield a surprisingly bountiful harvest.