I think you could do this a bit easier by using common events, maybe something like...If you water the plant, it turns on a switch that activates that trees common event and adds 1 to the variable Tree 1. In the common event you should have conditional branches. Something a little like this.
Conditional Branch, if variable Tree 1 = 1
Wait so many frames
Turn switch Feed Me! on
Turn switch Tree 1 watered off
Else
Conditional Branch, if variable Tree 1 = 2
etc etc
Keep going however many times you want until the plant's fully grown. The event itself will have different pages with the same commands, something like this.
Tree stage 1
Conditions - none
Show Text - Water the tree?
Show Choices Yes, No
When Yes - turn switch Tree 1 fed on, add 1 to variable Tree 1
Tree stage 2
Conditions - Variable tree 1 = 1
Show Text - Water the tree?
Show Choices Yes, No
When Yes - turn switch Tree 1 fed on, add 1 to variable Tree 1
Etc etc
I'm not sure if this is the kind of tree growing you want though, I could tinker around a bit and sort it out properly, this is all off the top of my head, hope it helps even a little bit :3