When gardening in the city, every inch counts. The smallest planter box size I recommend is 12 inches (30 cm) in depth and width. This minimum dimension is crucial for several reasons. First, it provides enough soil volume to retain moisture and nutrients, reducing the frequency of watering and fertilizing. Second, it allows adequate room for most herb and small vegetable root systems to develop properly. Plants like lettuce, radishes, basil, and thyme can thrive in boxes of this size.
For a single plant, a 12-inch pot is sufficient. For a rectangular planter box, ensure it is at least 12 inches deep and 6-12 inches wide per plant. Depth is especially critical; shallow planters dry out too quickly and stunt root growth. If you have space, opting for boxes 18-24 inches deep will support a wider variety of plants, including cherry tomatoes or peppers.
Material choice also matters with small sizes. Lightweight, durable materials like fiberglass, metal, or cedar are excellent for balconies and rooftops. Always ensure there are drainage holes to prevent waterlogging. By starting with the 12-inch minimum, you create a foundation for a healthy, productive urban garden, even in the tightest spaces.