A container for growing plants has a base and a side wall extending upwardly from the base, for containing a growing medium. An opening is defined in an inclined portion of the side wall, and a rib positioned on the side wall and facing into the container defines a root-guiding angle between the rib and the side wall, the root-guiding angle leading to the opening. Plant roots growing adjacent to the side wall are therefore guided by the root-guiding angle to the opening to be air pruned.