Yes, there is an urban planter box big enough for a small lemon tree, but it needs to be larger and more carefully set up than a typical herb box. For a dwarf or small lemon tree, choose a planter box that holds at least 15 to 20 gallons, with a practical minimum of about 24 to 30 inches long, 15 to 18 inches wide, and 15 to 18 inches deep. A slightly larger box is even better because lemon trees dislike cramped roots and dry out too quickly in tiny containers.
Look for a food-safe, UV-resistant planter box with generous drainage holes and pot feet or casters. Drainage is essential: citrus roots rot in soggy soil. Use a well-draining citrus or container mix rather than heavy garden soil, and place the box where the tree gets at least six to eight hours of direct sun. On a balcony, check the weight limit first, because wet soil is very heavy.
Water deeply when the top two inches of soil feel dry, then let excess water drain away. Feed with a citrus fertilizer during spring and summer, and prune lightly to keep the tree compact. If winters are cold, choose a box with wheels or move the tree indoors before frost. With the right planter box, sunlight, drainage, and care, a small lemon tree can thrive in an urban space.