A lawn mower (100) is described provided with a flap (102) arranged on a wall (103) of a bag (101) for collecting grass, said flap (102) being configured to lift up under the action of the air circulating inside the bag (101) when the lawn mower (100) is in operation and the bag (101) is not completely full, and also to lower down in the absence of air when the bag (101) is completely full. The lawn mower (100) comprises an apparatus (1) for detecting and signaling the bag (101) conditions, in turn comprising at least one proximity sensor (2) configured to detect a change in the flap (102) position and signaling means (6) adapted to warn the operator about the bag (101) conditions.
