For most urban root-vegetable planter boxes, 12 inches (30 cm) of loose, free-draining soil is the practical minimum. That depth is enough for radishes, salad turnips, green onions, and many compact beet varieties. If you want a more versatile planter that can handle most common root crops without limiting your choices, aim for 15 to 18 inches (38 to 45 cm). This extra depth gives carrots and parsnips room to extend downward, helps potatoes develop without crowding, and makes watering and root growth more forgiving in a small urban garden.
The right depth depends on what you plan to grow. Radishes and small turnips can succeed in 6 to 8 inches (15 to 20 cm), but a planter that shallow dries out quickly and limits crop rotation, so it is not the best choice for a main root-vegetable bed. Beets generally need 10 to 12 inches (25 to 30 cm). Carrots are the crop most likely to disappoint in a shallow box: round and baby carrots can grow in 8 to 10 inches, but standard and long carrots need at least 12 inches, and 15 to 18 inches is better for straight, well-formed roots. Parsnips are deep-rooted and should have 15 to 18 inches (38 to 45 cm). Potatoes are usually grown in 15 to 18 inches of soil, with extra soil or mulch added as the plants grow, while sweet potatoes prefer a generous 18 to 24 inches (45 to 60 cm) because they produce storage roots underground.
Remember that planter depth means the actual interior soil depth, not the outside height of the box. A box may look deep but lose several inches to drainage material, an air gap, or a false bottom. Use the full interior depth for soil, make sure there are drainage holes, and fill with a loose, stone-free mix rather than heavy garden clay. Root vegetables need air and water movement, so compacted soil will cause short, forked, or stunted roots even if the box is deep enough. A mix based on topsoil, compost, and coarse sand or grit works well for most urban planters.
If space is limited, you can absolutely grow root vegetables in a 12-inch-deep planter by choosing compact varieties, such as baby carrots, round beets, radishes, and small turnips. But if you want one reliable urban planter box for a wider range of root crops, choose 18 inches. It costs more to fill, but it gives you better moisture stability, more rooting room, and far more flexibility from season to season.