I like the idea, life needs to have more plants.
A limitation is the small set of decks this creature can be included within. Any other creature that is dual (FFQ, Lava golem, toadfish, etc...) can also be used within a rainbow deck. This creature *could* but with needing 8 life to use, then needing even more stored up to keep it from being too weak... it's not a good choice. That seems to be a limitation for this guy.
More on the mechanics side of things, does the change of life and attack effect it's maximum or it's current? I'd assume it raises it's maximum life / attack when it grows and then loses from it's current life / attack when it shrinks. How will poison or damage work towards it in that case? If it effects max, but not current, then I can have my tree poisoned, use quanta to bring it's max down, then gain life quanta to heal it back up? On the flip side, if it has lost some life, and I spend life quanta, will that kill it because it will shrink and it's life lost is greater than it's now current life? Having a genitive effect happen (creature dying) due to a positive action (playing cards) seems to be a bit bitter.