Yes, lavender can grow in an urban planter box, but it does need more space than many small herb containers provide. Lavender develops a deep, wide root system and hates sitting in wet, compacted soil, so a shallow or narrow planter box will usually lead to weak growth, poor flowering, or root rot. For one plant, aim for a container at least 12 to 16 inches deep and 12 to 18 inches wide, with drainage holes. If you want several plants, space them so air can move between them; crowded lavender is more prone to mildew and leggy growth.
Choose a compact lavender variety such as 'Munstead' or 'Hidcote' for balcony or patio planters. Use a gritty, free-draining mix rather than rich compost, and add perlite or coarse grit if needed. Place the planter in full sun for at least six to eight hours a day. Water deeply when the top inch of soil feels dry, then let it dry out again; lavender prefers slightly dry roots to constantly damp soil. In an urban setting, keep the box away from downpipes or shaded corners where water collects.
A little maintenance makes a big difference. Prune lightly after flowering to keep the plant bushy, but avoid cutting back into old, bare wood. In wet winters, raise the planter slightly or protect it from excessive rain, because cold wet roots are more damaging than cold alone. If your planter box is only 6 to 8 inches deep or very narrow, lavender is not the best choice; use a deeper box or a larger individual pot instead. With good drainage, strong sun, and enough root room, lavender can be a healthy and attractive urban planter plant.