In most urban settings, planter boxes with legs can help keep plants cooler than identical boxes sitting directly on hot pavement or concrete—but they are not automatically cooler. The key is where the heat comes from and how the planter manages it.
When a planter sits on the ground, it exchanges heat with that surface. On grass, soil, or shaded earth, the ground may be cooler and moister than the air, acting as a heat sink. This can buffer root-zone temperatures and slow moisture loss. On asphalt, concrete, or stone that has been baking in the sun, however, the surface can be much hotter than the air. A ground-sitting pot then receives conductive heat from below, and the root ball can get dangerously warm.
Legs break that direct contact. Air can move under the box, reducing conductive heat transfer from a hot surface. That airflow also helps drain excess water and can carry away some heat from the bottom. In dense cities with hot hardscapes, this is a real advantage, especially for heat-sensitive plants.
But legs introduce trade-offs. A raised box has more surface area exposed to the air, including its bottom. If the air is hot and dry, or if the box is made of dark metal or thin plastic in full sun, the root zone can heat up faster. The box also has less thermal mass connected to it, so it may swing more quickly between hot days and cool nights. Ground-sitting planters, by contrast, can stay more stable when the ground is cool and shaded.
The material and design matter as much as the legs. A light-colored, insulated, wooden, or thick plastic planter will stay cooler than a black metal one. A large soil volume holds more moisture and temperature stability. Mulch, shade cloth, companion plants, or a double-walled design can protect roots. Legs that are too short may simply place the box in hot, still air near pavement; taller legs with good cross-ventilation generally work better.
So the honest answer is: legs can keep plants cooler when the ground surface is a major heat source, such as sunbaked pavement. They may not keep plants cooler—and can even make them hotter—when the problem is hot air, intense sun, small soil volume, or a heat-absorbing planter. For urban gardeners, choose legs for hot hardscapes and airflow, but pair them with shade, light colors, insulation, mulch, and reliable watering. If the planter sits on cool shaded soil, ground contact may be an asset rather than a liability.