The present invention includes a plant holding unit for supporting a plant on a generally vertical member. A plant container held by plant holding unit is held below a plane approximately normal to the outer surface of the generally vertical member. The plant holding unit is configured to support a plant container on a generally vertical member. The plant holding unit includes an opening having an axis and a perimeter edge, wherein the axis is approximately perpendicular to the first plane defining a general direction of plant growth. The plant holding unit also includes a barrier extending generally vertically from and attached to the perimeter edge, wherein the barrier forms a water retaining area.