You need to line your planter box if it's made from wood or metal. The liner will help prolong the planter's life. You don't need to use a liner if the planter is made using plastic, ceramic, or concrete as they are quite durable by themselves.

Do you line a raised planter box?

Yes, you should line your raised garden bed, since the pros of doing so outweigh the cons. A liner for your raised garden bed can insulate the soil against extreme temperatures, keep moles and gophers out, and prevent weeds from growing.

Is it safe to line raised bed with plastic?

You can line your raised bed to make it more durable and to prevent toxics from leaching into the soil. For lining, use landscape fabric found at garden supply stores or cloth fabric from clothing. Avoid non-porous plastic, as it can retain too much water and discourage beneficial insects and worms.