In summer, there is no fixed watering schedule that works for every urban planter box. The right frequency depends on the box size, plant type, soil mix, sun exposure, wind, and local heat. As a general guide, small planter boxes in full sun may need water every day—and sometimes twice a day during a heatwave. Medium-sized boxes usually need water every one to two days. Large, deeply planted boxes may last two to three days, especially if they are mulched and partly shaded.
Do not water by the calendar alone. Check the soil first: push a finger about 2–5 cm (1–2 inches) into the soil. If it feels dry at that depth, water thoroughly. If it is still cool and moist, wait. A moisture meter can help, especially for large boxes.
When you water, apply enough to soak the root zone and let excess drain out. Morning is usually the best time because it reduces evaporation and gives plants time to recover before the hottest part of the day. Avoid light daily sprinkling, which encourages shallow roots. Mulch the surface, group pots to create shade, and use self-watering planters if you are often away. During very hot, windy weather, check your urban planter box once in the morning and again in the afternoon, but only water if the soil is dry.