This invention is an apparatus for a lawn mower having a rotating blade enclosed by a deck. A grass catching hopper is removably secured to the mower. A first conduit communicates with the deck or, in another embodiment, with a blower which induces increased airflow and moves grass clips and debris to the hopper interior. Controlled and enhanced air flow results from an air flow insert in the first conduit entrance. Air and entrained materials collide with a hopper interior wall, and a large amount of entrained materials drop to the hopper floor. Air and the small amount of entrained materials remaining in it are directed back toward the first wall and through an opening to a centrifugal chamber. Small openings in the chamber permit the passage of air flow and entrained materials into the hopper. Air and what remains of the entrained materials are then drawn out of the chamber and back to the intake of the blower through a second conduit.