Pick the bad apples early
But heads say it is local authorities that are failing to "bite the bullet". This is despite dismissals having been made easier by removing the requirement that teachers can only be sacked by a two-thirds majority of councillors on an education committee.
Glasgow tops the league with six dismissals after complying with the local negotiating committee agreement on competence.
"Let's be absolutely honest," Mr Gardner said in an interview with The TES Scotland. "This is not easy - it is not simply a referral from a headteacher to the director. It is hard for headteachers. There can be cases where the headteacher may have established a strong professional or personal relationship with the teacher, especially in small schools.
"The head will be conscious of the staffroom dynamic and may be concerned that confronting the issue of competence of a teacher may undermine the teamwork he or she may have worked hard to build up. The headteacher will also be aware of the family and financial commitments a teacher carries.
"However, heads must regard themselves as advocates for the children and young people and consider the impact that a poor teacher and poor teaching has on the future life chances of the pupils."
Mr Gardner refuted any suggestion the profession was rife with incompetents. "Indeed it is the opposite," he said. "But the impact of one poor teacher is devastating on those children who are taught by them."
He had recently dealt with cases of incompetent teachers who had considerable experience, yet no one had previously highlighted difficulties with their teaching. Reports from heads may have contained no references to weaknesses or difficulties.
"Headteachers have a collegiate responsibility to colleagues, and to the teachers themselves, to ensure that reports reflect on the performance of the teacher. The least demanding course of action for a head is to keep the classroom door shut and hope the problem will go away. Of course, it never does and problems simply increase."
His advice to heads was to intervene early and provide the support that could lead to improvement or take the necessary action before teachers become embedded in the system.
Mr Gardner acknowledged that measures to improve such teachers was demanding for heads. They have to put in place internal support and mentoring, visits to other schools or departments and opportunities for continuing professional development. On top of that is constant monitoring and the setting up of a formal interim and final review.
"Clear and comprehensive documentation requires to be maintained so that if a teacher has not met the improvement targets over the duration of the support programme - and where this is regarded as 'chronic incompetence' - then the evidence is there for the education authority to progress the matter through the discipline framework," Mr Gardner said.
He recognises union concerns about the need for "due process" to be seen to be done. Authorities cannot ignore employment law and must be seen to treat employees reasonably. In Glasgow, once heads refer teachers to the authority for disciplinary action, dismissal follows.