For most urban growers, the best urban planter box for lettuce and spinach is a self-watering planter box that is 8 to 10 inches deep, 12 to 18 inches wide, and made from food-safe, UV-resistant material. Lettuce has shallow roots, while spinach needs a little more depth and steady moisture, so this size gives both crops enough room without wasting balcony space. Look for a rectangular urban lettuce planter with drainage holes and an overflow reservoir. The reservoir reduces daily watering, and the drainage prevents soggy roots after heavy rain.
The best all-around choice is a rectangular self-watering planter with an 8–10 inch soil depth, a 12–18 inch width, and a light-colored food-safe HDPE, cedar, or glazed ceramic body. A balcony vegetable planter should also be stable, easy to move, and narrow enough to fit a railing or wall. Avoid dark, thin plastic boxes that overheat in full sun; lettuce prefers cool roots, and spinach bolts quickly in heat. If your balcony gets strong afternoon sun, choose a light color or place the planter where it receives morning sun and afternoon shade.
For a smaller space, a 7–10 gallon fabric grow bag works as a budget-friendly shallow-root planter, but it dries out faster than a self-watering box. For spinach, choose a spinach planter with at least 8 inches of soil depth and consistent moisture; spinach is less forgiving of dry spells than lettuce. For hot rooftops, an insulated or light-colored self-watering planter box is worth the extra cost because it keeps root temperatures lower.
Use loose potting mix rather than garden soil, make sure excess water can escape, and sow lettuce about 4–6 inches apart and spinach about 3–4 inches apart. Water consistently, harvest outer leaves, and keep the planter in partial shade during hot weather. Final recommendation: choose an 8–10 inch deep, 12–18 inch wide self-watering rectangular planter with drainage and overflow; it is the most reliable urban planter box for growing lettuce and spinach on balconies, patios, and small rooftops.