A planter for growing a transplantable plant includes a container having a sidewall and a bottom and further includes a retainer member for retaining a plant which has been transplanted within the container through the sidewall or bottom thereof. The retainer member defines an expandable opening which is movable between expanded and normally-collapsed conditions so that by urging a plant root system end-first against the retainer member, the expandable opening is moved to an expanded condition to permit the root system to pass through the expandable opening into the container interior. Upon passage of the root system through the expandable opening, the expandable opening is permitted to return toward its normally-collapsed condition about the stem of the plant to thereby prevent the withdrawal of the root system of the plant from the container interior through the plant retainer.