For a typical sunny urban planter box, the most low-maintenance plant is sedum, also called stonecrop—especially low-growing varieties such as Sedum spurium, Sedum acre, or creeping sedum. These plants are almost made for the tricky conditions of city containers: shallow soil, reflected heat from walls and pavement, strong wind, irregular watering, and long stretches of neglect.
Sedum wins because it is drought-tolerant, shallow-rooted, and very forgiving about soil. It does not need rich potting mix, frequent feeding, deadheading, or constant pruning. It handles heat and poor, gritty soil far better than most flowering plants, and it usually stays healthy without sprays or special care. In mild climates, many sedums keep some color or foliage through winter, and in summer their small starry flowers attract pollinators.
To keep sedum nearly fuss-free, use a container with drainage holes and a well-draining mix—ordinary potting soil blended with perlite or coarse grit works well. Plant it in full sun for at least six hours a day, water deeply only when the soil is dry, and avoid keeping it soggy. A light trim in spring is usually all it needs to stay compact. Do not overfeed it; too much richness makes sedum soft and floppy.
If your planter box is in deep shade, sedum is not the right choice, and no sun-loving plant will be truly no-fuss there. In that case, pothos or spider plant may be better, though they need more consistent moisture. But for the classic sunny balcony, rooftop, or street-facing urban planter box, sedum is the plant that asks the least and gives the most.