There is no single watering schedule that works for every urban planter box. The best rule is to check the soil regularly and water when the top 1–2 inches feel dry. In warm, sunny, or windy weather, planter boxes can dry out very quickly—sometimes needing water every day, or even twice a day for small boxes with thirsty plants. In cooler, cloudy weather, you may only need to water every two to five days.
Water deeply rather than giving a small daily splash. Apply water slowly until it runs out of the drainage holes, then empty any saucer so roots do not sit in water. Morning watering is usually best because it gives plants moisture before the heat of the day and helps foliage dry. Avoid watering only the surface, since this encourages shallow roots and weak plants.
Plant type matters too. Leafy greens, herbs, and young seedlings usually need more frequent checking because their roots are shallow. Tomatoes, peppers, and other larger vegetables may prefer less frequent but deeper watering. Small containers dry faster than large ones, and dark or metal planter boxes can heat up in the sun and lose moisture quickly. Using mulch on the soil surface can help slow evaporation, and self-watering planter boxes can reduce daily maintenance, though you should still check the reservoir and topsoil.
A simple routine is to check your planter box every morning by touching the soil or using a moisture meter. Water when the top layer is dry, adjust for the weather and plant size, and watch your vegetables for signs of stress. If leaves wilt in the afternoon but recover by evening, that can be normal heat stress; if they are wilted in the morning, the plants likely need water. By checking the soil instead of following a fixed calendar, you will keep urban planter box vegetables healthier and avoid both underwatering and overwatering.