There is no one person, or ten people who are the most influential in garden retail. The most influential people in garden retail are those who are involved in sharing with one another, mostly via the numerous online trade groups. It's the ability to share ideas in real time with others in the trade, no matter where they are located.
Some of the best ideas I have heard are from people who otherwise wouldn't even be known outside their garden shop world. They willingly share their ideas for the rest of us to try. I look forward to what my fellow trades people have to say. It's through the folks who are on the floor working with the customer that the most profound ideas arise.
My vote for the most influential people in garden retail is a vote for the tradespeople who willingly share their ideas with others without any compensation other than the joy of sharing.