In an urban planter box, you will usually need to water more often than in a garden bed. A garden bed has a large volume of soil and better contact with the surrounding earth, so it holds moisture longer. A planter box has limited soil volume, more exposed sides, and often faster drainage, so it dries out quickly, especially in sun, wind, or heat. As a general guide, check garden beds every two to three days in warm weather and about once a week in mild weather, while urban planter boxes may need water every one to two days in summer. Small or shallow boxes may need daily watering. During hot, dry, or windy spells, a small planter box may even need a second check in the evening.
Do not water by calendar alone. Push a finger about one to two inches into the soil. If it feels dry at that depth, water deeply until water runs from the drainage holes; if it is still moist, wait. Garden beds often benefit from a slow, deep soak once a week, while planter boxes need more frequent but still thorough watering so the entire root zone gets moist. Mulch both, but mulch is especially helpful in planter boxes because it slows evaporation. Choose larger planter boxes when possible, group plants with similar water needs, and use self-watering or moisture-retentive options if you cannot check daily. In rainy periods, reduce watering and make sure drainage works. In winter, water much less often, but do not let boxes dry out completely if plants are still growing. In short: expect urban planter boxes to need watering roughly two to three times as often as garden beds, and always let soil moisture, plant type, weather, and planter size guide you.