Ben Malin of Terraqueous (representing the Somerset Peat Producers) has been elected as the vice chair.
For Harper to become chair the GMA had to change its constitution to allow a non-member to take the role.
Harper said: "I am delighted to have been elected to chair the GMA for the next two years. Having recently moved to Gardman after working directly for growing media manufacturers for the past 15 years and being involved in the project to responsibly source and manufacture Growing Media I’m in a great position to be independent and yet fully understand the demands of the industry."
Working together with partner organisations, Projects 4 (Responsible Sourcing and Manufacture of Growing Media) and 7 (Performance Standards for Amateur Products) of the Growing Media Taskforce will be completed and turned into practical tools for the industry.
The GMA also plans to update its website incorporating both the GMA and John Innes websites into one, creating a resource for both retailers and consumers.
Harper said: "We plan to present the workings of Project 4 at a seminar at the end of June to all manufacturers, retailers, NGOs and other interested parties, with various manufacturers showing their findings."