Harper, formerly of Sinclair, Vital Earth and Bord na Móna, joined Gardman in January from Bord na Móna as sales director with remit to focus on independent garden centres.
He said: "I've got a few small projects that have been bubbling under for ages and ages that I have never had time to progress. I'm working on a start-up for somebody and consulting for somebody else. Then I'll decide what I'm going to do."
The Growing Media Association chairman said Gardman had "moved forward massively" in the six months he had been there. He added that he had brought in a new sales structure.
Jason Richardson and Lolly Lee now run Gardman sales.
Private equity firm Rutland Partners put £5m into Gardman in July as part of a management buy out.