Power
Wholesale nurserymen sometimes get to influence market trends by growing and marketing plants before retail nurseries and landscapers ask for them. Really, though, it's the landscapers who have the most influence. They plant all the things that people see, think are pretty, and want. Which is why there are so many London plane trees and crape myrtles out there.
There's really not much power in the wholesale nursery industry. Believe it or not.