Gwyther will be spending more time at home and in France, whilst taking on some advisory and non-executive roles.
HTA president Carol Paris said: "David has made a huge contribution to the horticultural industry in the 16 years that he has been at the helm of the association and it has flourished under his leadership.
"David’s guidance has seen the HTA develop a wide range of services and benefits for the members, including increasing gift voucher sales, improving training for retailers and growers, effective government lobbying and building excellent relationships with members and partners."
She added that no successor is in place but the HTA will be advertising for a new director general soon.
Horticultural Development Company chair Neil Bragg said he was surprised to hear of Gwyther’s retirement and that he would be difficult to replace.
"He’s been a very positive lobbyist on behalf of the industry," commented Bragg who also said he was concerned at the timing given the many challenges facing the industry right now.
"He’s been involved with the National Horticultural Forum that has taken itself to a new level, and a lot of the drive to do that has come from him," he added.
Grower Derek Jarman said: "He has done a solid job and represented garden centres and growers for the benefit of the industry. But I would have expected him to say he was going to retire next year to give time to find a replacement."