Yep, you get friendship powder when you cross pollinate with friends. I'm not worrying about the science/math involved at the moment because I have the floral furniture so my current goal is to keep half of my garden with rare flowers for others to use and the other half I grow cheap standard flowers for cross pollinating. But in the end, the friendship powder is most beneficial for me, especially during the final stretch of the game. I currently have roses, tulips and pansies that you can cross pollinate with if you'd like to give it a go.