Work on getting a number of high level friends.
Personally, I like to water those with whom I have a high friendship level, cause top 3 I use for the commission thing and by focus on watering a few ppl of high relationship, it's more likely that it goes toward future top 3s.

Also, for the harvest exp, friendship level increases the amount of exp gained by water. I think it's every 3 levels or something. So if a lv 13 friend water you, you get like 5 exp, which is a quarter way done even if u have high lv tree. Just a few of those will get your tree grown.