For basil and mint, a good rule of thumb is to provide at least 8–10 inches (20–25 cm) of actual soil depth in an urban planter box. If you have the space, 12–18 inches (30–45 cm) is better because it gives roots more room, holds moisture more steadily, and buffers against hot, windy city conditions.
Basil is usually the more demanding herb of the two. It prefers rich, well-drained soil and consistent moisture, and a planter box that is only 6 inches deep can dry out too quickly on a sunny balcony or rooftop. A depth of 10–12 inches is a practical minimum for healthy basil, while 12–15 inches is ideal if you want stronger, more resilient plants.
Mint is more forgiving about depth because its roots are relatively shallow, but it spreads aggressively through runners. In a shallow box, mint can become root-bound quickly and may dry out faster. A depth of 8–10 inches is usually enough for mint, but the bigger issue is width and containment. Grow mint in its own section, a separate pot, or behind a root barrier so it does not take over the planter box.
If you are growing basil and mint together, choose a planter box that is at least 10–12 inches deep and wide enough to give each plant space. Make sure the box has drainage holes and use a free-draining potting mix rather than heavy garden soil. In urban planters, drainage and soil volume matter as much as depth: a deeper box with good drainage will stay healthier than a shallow box that holds water or dries out unpredictably.
In short, do not go below 8 inches for mint and 10 inches for basil. For the easiest results, aim for 12 inches or more of soil depth, keep mint contained, and make drainage a priority.