The most reliable way to keep weeds out of an urban planter box is to prevent them from getting a foothold in the first place and then deal with any seedlings early. Start with clean growing media: use a quality potting mix rather than scraping up garden soil, and choose compost that is fully composted. Garden soil often carries weed seeds, roots, and rhizomes that can be very difficult to remove from a contained planter box. If you must use garden soil, keep it to a small proportion and screen it carefully, but clean potting mix is usually the safer choice.
After planting, add a layer of mulch two to three inches deep. Straw, shredded bark, leaf mold, or fine wood chips work well, but leave a small gap around stems so the mulch does not smother plants or hold moisture against them. Mulch blocks light, which stops many weed seeds from germinating, and it also helps the soil retain moisture so you are not watering as often. In a small urban planter, a neat mulch layer is one of the easiest and most effective defenses.
Plant densely but sensibly. If your planter box has bare soil, weeds will fill the gap. Use vegetables, herbs, or annual flowers to cover the surface, but leave enough airflow to reduce disease pressure. Water at the base of plants rather than spraying the whole surface; wet mulch and exposed soil can encourage weed seeds near the surface. Check the edges, corners, and drainage holes of the planter box regularly, because weeds often appear first in these overlooked spots.
Pull weeds when they are small, ideally before they flower and set seed. A hori-hori knife, a narrow weeding tool, or even a sturdy dinner knife can help loosen roots in a tight planter box. Try to disturb the surrounding soil as little as possible, because turning the soil can bring buried weed seeds to the surface. If you see weeds growing around the outside of the box, clear them too, since they can drop seeds into the planter. With a little routine attention, clean soil, mulch, and early hand-pulling, an urban planter box can stay tidy without relying on harsh chemicals.