Love the idea. I’m always into gameplay involving plants 
Before visiting your github page and reading the README I was trying to envision what a competitive tree growing game might look like.
My first thought was for 2 or 3 players, or 2-3 teams. There would be a central “soil” tile and each team would place one tile as an initial trunk sprouting off of that towards them. For 2 players/teams they would be placed on opposite sides of the soil. For 3 players/teams they would be evenly spaced around the soil.
Gameplay proceeds from there to grow your tree. Having everyone connected to the same constellation of tiles makes win conditions easier and allows for one team to directly impact another team.
The downside is that you are growing your tree towards yourself, which is totally upside down relative to what you’d expect. So maybe not the best idea!
Can’t wait to see what you come up with.
[EDIT] Also, the plant growing algorithm in Terrarium is somewhat simplified in order to conserve code and data space. I’m excited to see what a game dedicated to the idea might add gameplay-wise.