Not automatically—but it can, especially in hot, sunny sites. Metal conducts heat quickly, so dark, thin-walled, small containers in full sun can push soil temperatures above what many roots prefer. The risk is highest when the planter is against a wall or on paving that reflects heat, when it has little soil volume, and when the soil is dry. Roots and soil microbes can suffer heat stress, and you may see wilting, poor growth, leaf scorch, or root damage even with regular watering.
However, metal is not a problem by itself. A well-designed metal urban planter box can stay within a safe range if it has generous soil volume, a light or reflective finish, drainage, and some insulation. Double-walled planters, air gaps, liners, cork or foam layers, and shade help buffer the root zone. Mulch reduces surface heating and moisture loss; drip irrigation keeps soil cooler through evaporation. In very hot climates, choose heat-tolerant plants or use metal as a decorative outer shell with a separate inner pot.
To check your planter, measure soil temperature 10–15 cm (4–6 in) below the surface at midday or late afternoon. If it stays above roughly 30–32°C (86–90°F) for long periods, most common urban plants will be stressed. Move the planter to afternoon shade, add insulation, paint it a lighter color, or plant in a larger volume. With those steps, a metal planter can be a good, durable home for plants rather than an oven for roots.