Short answer: yes, a dark-colored urban planter box can overheat plants in direct sunlight, especially in hot weather, small containers, or metal and plastic walls. The risk is less about leaves instantly “cooking” and more about the root zone getting too hot. Dark surfaces absorb more solar radiation than light ones and convert it into heat. In a planter, that heat radiates inward to the soil and root ball. If the box is small, has thin walls, sits on hot concrete, and receives strong afternoon sun, soil temperatures can climb well above the surrounding air temperature. Many plants suffer when root-zone temperatures stay above roughly 30–32°C (86–90°F), and sensitive roots can be damaged above 35–38°C (95–100°F). Symptoms include wilting despite moist soil, yellowing, stalled growth, scorched leaves, flower drop, and increased root rot risk if heat is combined with overwatering. Seedlings and shallow-rooted plants are most vulnerable.
However, dark color alone does not guarantee disaster. A large, thick-walled planter has more soil volume and thermal mass, so it heats more slowly. Healthy plants with dense canopies shade their own soil. In cool or maritime climates, dark containers may even be beneficial early in the season by warming soil. The real danger is a combination: dark walls, direct midday or afternoon sun, small volume, poor airflow, reflective walls or pavement, and a heat-sensitive plant.
To reduce risk, choose lighter colors or wrap and paint the sunny side a light reflective color. Shade the planter body—not necessarily the plant—with a breathable cloth, reed screen, or nearby pot. Add mulch to insulate the soil surface. Group pots so foliage shades them, or move them to morning sun and afternoon shade. Use larger containers, double-pot with an air gap, or insert a lighter inner pot. Avoid dark metal in hot sun unless insulated. Check soil temperature with a probe; water deeply when needed, but do not keep soggy soil. If roots are already hot, move the planter to shade, shade the pot, and water gradually with cool—not ice-cold—water. In short: dark planters can overheat roots and stress plants in direct sun, but smart placement, insulation, and plant choice can keep them from cooking.