Short answer: you can, but it is usually not the best first choice in an urban planter box. Direct composting—often called in-situ composting—means burying kitchen scraps or adding compostable materials straight into the soil where plants are growing. It can work in a large, deep raised bed with mature plants, good drainage, and careful management. In a small balcony planter, however, it can go wrong quickly.
The main problem is that an active compost pile and a healthy plant root zone want different conditions. Compost needs a mix of greens and browns, air, and moisture, and it heats up as microbes break materials down. Plant roots need stable, well-aerated soil. If you bury too many food scraps in a confined planter, the material can turn anaerobic, smell, attract fruit flies or rodents, and create pockets of ammonia or heat that stress roots. Small volumes also dry out or stay soggy faster, and urban planter boxes often have limited drainage and close neighbors.
That does not make direct composting a terrible idea in every case. It can be a reasonable low-effort method if you treat it as slow, buried trench composting rather than as a hot compost pile. Use a large planter or raised bed, not a tiny balcony box. Bury only small amounts of fruit and vegetable scraps deep in one corner, away from main roots, and cover them with soil. Add dry brown material such as shredded paper, cardboard, or mature compost. Keep scraps covered, avoid meat, dairy, oils, cooked food, and pet waste, and do not let materials pile up above the soil. Rotate burial spots and give each addition time to break down.
For most urban gardeners, a separate system is safer and more efficient. A lidded compost bin, worm bin, or bokashi bucket keeps food scraps out of the planter while you grow plants. You can then use the finished compost as a soil amendment. If you want the convenience of in-planter composting, consider a worm tower or a removable composting insert designed for raised beds. These keep the active decomposition zone contained and reduce the chance of root damage and pests.
So, is composting directly in an urban planter box a terrible idea? Not automatically—but it is a poor fit for small containers and beginners. Use direct composting only in a large, well-drained planter with careful burial and modest amounts of scraps. Otherwise, compost in a separate bin and add the finished product to your planter. That approach is cleaner, more reliable, and much less likely to annoy your plants or your neighbors.