Thank you very much for your feedback and appreciation of the game! Unfortunately we ran out of time on creating a tutorial. Currently you can level up the plants by merging 3 of the same type together! And it's a power of 3 continuously on growing them in power. Each time you merge 3 of the same tier, they get stronger and stronger!
Here is an example of the merging mechanic if you wish to give the game another go!
Thank you again!