Metal planter side walls in direct sun can absorb and conduct heat. If the roots of the plant come into contact with hot metal it is possible for the roots to become damaged. To remedy this issue you can place your planter box in a shady area, or if the size of the box allows you can simply plant your plants a few extra inches away from the side walls to avoid contact with the potentially hot metal. Additionally, regular watering will help to keep the soil and overall temperature of the planter box cool.
When using metal planters in an area with prolonged direct sun, which can heat the planter side walls, it is prudent to take steps to insulate the perimeter of the container. All Nice Corten Steel Planters ship with a polypropylene twin wall insulating liner, which protects the roots of your plants by forming an air gap on all four sides of the box . The food grade polypropylene liner is BPA-free and will not leach chemicals into the soil. If you do not have one of our planters, this material is available at Home Depot or Lowes by sheet and can be easily cut with a utility knife to fit your planter.
For a DIY approach, using scrap cedar siding or fence boards will result in a durable organic liner that will keep your plants cool for many years. Cedar is naturally rot resistant and will last for many years. Further, the smell of cedar oil naturally repels insects and pests.
Below is a video of the two options