Northampton star Hartley, 26, was found guilty of biting the finger of Ireland flanker Stephen Ferris in England's 30-9 RBS 6 Nations victory at Twickenham.
The independent disciplinary panel ruled Hartley's reaction to finding Ferris' finger inadvertently in or near his mouth was "not justifiable" and banned him until May 14.
Hartley could have faced a much lengthier suspension given he was banned for six months in 2007 for eye-gouging Wasps players Jonny O'Connor and James Haskell in the same match.
But the panel decided to "ignore" the gouging incident - and they reduced the ban from a potential 12 weeks - following "glowing" character references from Rowntree and Northampton director of rugby Jim Mallinder.
Hartley's eight-week suspension means he will be available for the Aviva Premiership final at the end of May, if Northampton qualify, and for England's summer tour of South Africa.
Rowntree, the England forwards coach, insisted on attending the hearing to speak on Hartley's behalf and detailed the "massive change" he has seen in the player since his "struggle in 2007".
The full written judgement details Rowntree describing Hartley as "a beacon and a leader" during England's recent troubled times and he added "no-one had more credibility with his peers".
Harlequins flanker Chris Robshaw was made England captain for the Six Nations but Rowntree said Hartley "was very close to having been made captain of the national team and might well soon become captain".
Rowntree finished by saying Hartley's reputation is "completely unfounded" and that "if you look at the many videotapes of the matches in which he has played you will see a man of humility and respect".
Mallinder, who made Hartley the Northampton captain, also detailed how he felt the player had matured in the five years since his previous suspension.
Hartley told the disciplinary panel he was a different person to the "loose" and "unpredictable" character from 2007, having been helped by sports psychologist Steve Peters and Northampton forwards coach Dorian West.
The panel accepted Rowntree and Mallinder's plea for them to recognise Hartley as being "very different from the man who committed the offence in 2007".
The full 13-page report, signed by chairman Roger Morris, says: "The easy course for the committee to follow was to ignore what they had heard and take the player's previous suspension into account.
"However to do that would have been to rely on the anecdotal baggage surrounding the player and not on the facts before them.
"It would also fail to acknowledge that the purpose of sanction is also to cause an offender to think again about his behaviour and that, the committee felt compelled to conclude, was what the player had done."
The three-man disciplinary committee acknowledged Hartley's disciplinary record had been clean for five years since the gouging incident and reduced his ban from a potential 12 weeks.
The report concludes: "The committee considered it appropriate to give the player credit for his clean (apart from one instance they had decided to ignore) record, his exemplary demeanour at the hearing, the glowing testimony from Messrs Rowntree and Mallinder and the compelling story they had heard about how the player had been prepared, himself, to address and seek to correct his own behaviour."
The panel concluded Ferris' finger was in or near Hartley's mouth as a consequence of his attempts to clear the England player from a ruck with a judo throw.
The committee found it "difficult to envisage a situation in which a bite would not be an intentional action".
They were prepared to accept Hartley's was "a reaction to a situation in which the player found himself" but stressed it was not justifiable and imposed the ban until May 13.
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