In summer, there is no single watering schedule that works for every urban planter box. A small, shallow terracotta box in full sun on a hot balcony may need water once or even twice a day. A larger, deeper plastic or resin planter in partial shade may need water every one to two days. During a heat wave, many urban containers need a daily check and sometimes a second watering in the evening.
The safest rule is to check the soil instead of following a fixed calendar. Push your finger about 1 to 2 inches, or 2.5 to 5 cm, into the soil. If it feels dry at that depth, water thoroughly until water runs out of the drainage holes. If it feels cool and moist, wait and check again later. You can also lift the planter; a surprisingly light pot usually means it is dry.
Water in the early morning when possible. This gives plants time to absorb moisture before the midday heat and reduces evaporation. If plants are badly wilted in the evening, water again slowly and deeply. Avoid frequent light sprinkles, because they encourage shallow roots and leave deeper soil dry. Use mulch, saucers, self-watering inserts, or drip irrigation to help stabilize moisture. Group pots together and move them out of the hottest afternoon sun if you can.
During an average summer, plan to check urban planter boxes every morning. Most will need watering about once a day, but expect more often for small pots, full sun, windy balconies, and thirsty vegetables like tomatoes, cucumbers, and basil. Cacti, succulents, and drought-tolerant herbs may need water only every two or three days or less, depending on conditions. When in doubt, feel the soil, check the drainage, and water deeply rather than often.