Session files not being deleted

My sessions directory just keeps growing in size until my website runs out of disk space. Why aren't old session files getting deleted? These are my settings, am I doing something wrong? Why isn't PHP garbage collection cleaning these out? Should I do it manually? Thanks

$config['sess_driver'] = 'files';
$config['sess_cookie_name'] = 'mywebsitesession';
$config['sess_expiration'] = (60 * 60); 
$config['sess_save_path'] = FCPATH . "sessions";
$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = TRUE;