The invention provides a spool cartridge (26) for arranging in use in a cutting head (20) of a string trimmer, in which the cutting head includes a locking arm (28) having an engagement end and a balance end, the spool cartridge comprising: a spool (36) rotatably mounted within the spool housing (34) and having cutting line wound thereon a spool housing having one or more openings (48,50) for cutting line a control gear coupled to the spool a locking assembly (60) in geared engagement with the control gear, the locking assembly being arranged to be engaged by the engagement end of the locking arm to automatically lock and unlock the spool with respect to the spool housing. In addition, a cutting head for a string trimmer and a string trimmer are provided, including such a spool cartridge.
