Thank you very much for trying the game! We are happy to know that you enjoyed playing it!
You can currently merge plants with no limit currently, like this:
- 3 of default tier 0 = 1 plant-turret of tier 2 .
- 3 plants of tier 2 merged together = 1 plant-turret of tier 3. And you can go even further like this, currently!
Hope it helps! We plan on continue adding features and fixing current issues with the game, after voting ends :D