A reciprocating plunger of a square baler has a series of cleanout probes fixed to an oscillating drive arm which reciprocates the plunger. The probes sweep along a path adjacent portions of a horizontal frame member of the plunger as the plunger moves through its operating cycle in order to remove the accumulation of loose crop materials from the same. The probes are arranged to swing away from the path of travel of needles which carry baling twine or wire upwardly through the plunger and toward an overlying knotting device, to thereby avoid interference with the needles and substantially prevent the needles from carrying any loose crop materials toward the knotting mechanism.