An automatic-feed trimmer head for a vegetation trimming machine. A spool is positioned within the interior of a housing. A cam guide formation is disposed atop the top flange of the spool. The cam guide formation defines a pathway. A cam arm is provided that is mounted to the housing with a pivot connection. The cam arm has a cam projection that extends into the pathway. A spring is provided that biases the cam arm into a first orientation. In the first orientation, the shape of the pathway enables the cam projection to move only in a first direction. The cam arm moves about the pivot connection from its first orientation to a second orientation when the trimmer head assembly is rotated above a predetermined rotational speed. When the cam arm is in its second orientation trimmer line is unwound from the spool.
