For salad greens, the minimum practical soil depth in an urban planter box is about 15–20 cm (6–8 inches). This is enough for shallow-rooted crops such as lettuce, arugula, spinach, and baby leaf mixes, especially if you harvest them young or use a cut-and-come-again method. However, 20–25 cm (8–10 inches) is a better target for most urban growers. A slightly deeper box holds moisture more evenly, gives roots more room, and makes the plants less likely to suffer during hot or windy weather. If the planter is in full sun or dries out quickly, choose the deeper end of that range. Make sure the box has drainage holes and use a light, fertile potting mix rather than heavy garden soil. In short, 15 cm is the absolute minimum, 20 cm is a safer minimum, and 25 cm is ideal for healthy, productive salad greens in an urban planter box.
What's the minimum depth for an urban planter box to grow salad greens?
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