The best urban planter box for a small lemon tree is a large, mobile, self-watering planter with excellent drainage and a light-colored, UV-resistant body. Aim for at least 20–25 gallons, roughly 24–30 inches wide and 18–24 inches deep. A dwarf or semi-dwarf lemon such as a Meyer lemon needs room for its roots and enough weight to stay stable in wind. A box that is too small dries out quickly, heats up, and can stunt fruit production.
Look for these features: food-safe material, drainage holes plus an overflow valve, a water reservoir that keeps soil evenly moist without waterlogging, breathable sides, and casters with locking brakes. Light-colored resin or double-walled plastic is often best for balconies and rooftops because it stays cooler and is easier to move. Untreated cedar or redwood is attractive and breathable but heavier. Powder-coated metal can work in cool climates, but it may overheat in full sun. Fabric beds drain well, but they dry out fast and may need more frequent watering.
For most urban growers, a 25-gallon light-gray resin self-watering planter on wheels is the best all-round choice. Fill it with a coarse citrus potting mix, place it in full sun, and water when the top inch or two feels dry. Use a slow-release citrus fertilizer in spring and summer, protect the tree from frost, and repot every two to three years. If you have a very sunny, hot balcony, choose a light color and add mulch. If winters are cold, choose wheels so you can move the lemon tree indoors or against a sheltered wall. The best planter box is therefore the largest one your space and budget allow, with drainage and mobility built in.