Chief executive Peter Rush said: "Danny has been instrumental in making Sinclair a well-respected and progressive horticultural business, one that is seen to be good to do business with.
"As a result the business has grown and expanded into new categories."
"We thank Danny for this and wish him every success in the future."
Adamson said: "I have thoroughly enjoyed my time at Sinclair and have relished working within the horticultural industry.
"Our division has made some excellent progess over the past six years and now it is well-placed to take the next step."