In most cases, a metal urban planter box will not get hot enough to literally cook plant roots, but it can create a warmer, harsher root zone than many gardeners expect. Metal conducts heat quickly, and a dark metal box in full summer sun can feel painfully hot on the outside. The soil inside, however, has mass and moisture, and roots usually grow deeper than the sun-facing wall. That buffering effect is why many metal planters perform well when they are large, well watered, and planted densely.
The risk rises when several factors combine: a shallow or small box, black or dark metal, full-day sun, reflective pavement or a heat-radiating wall, and inconsistent watering. In those conditions, soil near the metal wall can become too warm for sensitive roots, and plants may show midday wilting, slow growth, scorched leaves, or poor flowering. The roots closest to the hot side are the first to suffer; the whole root system is rarely cooked unless the planter is very shallow and exposed.
To protect roots, choose a light-colored or galvanized finish, or shade the sun-facing side with a trellis, shade cloth, or nearby plants. Larger soil volume helps, as does mulching the surface and watering deeply and consistently. A drip system or simple timer can keep moisture stable. If the box sits on hot concrete, raise it slightly for airflow. Some gardeners add an inner liner or wooden frame to create an air gap between soil and metal, but make sure drainage still works.
Check the soil temperature on a hot afternoon with a probe. If it feels hot to the touch and stays above the comfortable range for your plants, add shade or move the box. In short, metal does not automatically cook roots, but it demands thoughtful placement, watering, and heat protection in summer.