Yes, you can absolutely grow a lemon tree in a large urban planter box, but it will be more like caring for a miniature orchard in a controlled space than planting a tree in open ground. The secret is choosing the right variety, giving it enough root room, and staying consistent with sun, water, and feeding.
Start with a dwarf or semi-dwarf lemon. Meyer lemon is the most forgiving for containers, followed by Dwarf Eureka or Dwarf Lisbon. A large planter box should be at least 18 to 24 inches deep and wide, and ideally 20 to 30 gallons or more. Citrus have shallow but wide roots, so a box that is broader than it is tall often works better than a narrow, deep pot. Make sure it has drainage holes; lemons hate sitting in soggy soil.
Use a well-draining citrus or cactus mix, not heavy garden soil. A mix of pine bark, coarse sand or perlite, and compost works well. Keep the graft union above the soil line. Place the box where it gets at least 8 hours of direct sun. In urban settings, balconies and rooftops can be windy, so choose a sheltered spot and secure the planter. If the box is on a balcony, check weight limits; wet soil and a mature lemon tree can be very heavy.
Water when the top 2 to 3 inches of soil feel dry. In hot weather, that may mean watering every few days; in cool weather, much less. Water until it runs out of the drainage holes, then empty any saucer. Feed with a citrus-specific fertilizer from spring through early fall, following the label. Too much fertilizer can burn roots; too little causes pale leaves and poor fruit.
Lemons are mostly self-fertile, so one tree can fruit. In an urban planter, insects may be scarce, so you can hand-pollinate by moving pollen from flower to flower with a small brush. Fruit usually appears after two to four years, depending on variety and care.
Cold is the main limit. Meyer lemons tolerate light frost better than many citrus, but most lemons need protection when temperatures drop near freezing. Move the planter indoors, into a greenhouse, or wrap it with frost cloth. Indoors, give it the brightest window possible or use a grow light. Watch for scale, aphids, spider mites, and leaf miners. Wipe leaves, spray with insecticidal soap, or use horticultural oil as needed.
Finally, plan to repot or root-prune every two to three years. Container lemons can live for years, but they need refreshed soil and a little root space management. If you give them sun, drainage, food, and winter care, a large urban planter box can produce fragrant flowers and real lemons, even in the middle of the city.