URBAN PLANTER BOXES

For basil and mint in an urban planter box, plan for at least 12 inches (30 cm) of actual soil depth. Basil grows best with about 10–12 inches (25–30 cm), while mint is a vigorous spreader that prefers 12–18 inches (30–45 cm). If you want one box to suit both herbs, choose 15–18 inches (38–45 cm) of internal soil depth. This gives roots room to grow, helps buffer moisture on hot balconies, and reduces the need to water twice a day. A shallower 8-inch box can work for basil in a pinch, but it dries out faster and limits root growth. Mint in 8 inches is possible, but it will need more frequent watering and strict containment. Measure the internal depth from the bottom of the box to the soil line, not the outside height. Leave 1–2 inches of headspace below the rim for watering and mulch. Drainage is just as important as depth: use a box with drainage holes and a free-draining potting mix rather than garden soil. Because mint spreads aggressively by runners, plant it in its own container or use a sturdy divider inside a shared planter box; otherwise it can crowd basil. For a productive urban herb box, 12 inches is the practical minimum, and 15–18 inches is the comfortable sweet spot for basil and mint together.