A concrete block structure useful in forming a retaining wall having upwardly open openings to facilitate planting plants therein. The block has an upright front wall, laterally spaced side walls joined to the front wall, and an upwardly open interior, the side walls having upwardly open and laterally aligned grooves formed in them, and a rod received within the grooves and protruding laterally beyond each side wall. A flexible anchor sheet having a forward edge portion may extend along and above at least two adjacent blocks of the course, the anchor sheet being attached to the rod and extending rearwardly over an upper surface of the rear wall for anchoring reception in an embankment against which the blocks are placed.