That's a great idea! I thought about it while doing the game, but chose not to implement that, so flowers don't grow if someone says something hurtful like "you are not pretty". However, as you say, I also found myself trying to write full sentences to the flowers haha so I'll try to figure out something. Thanks for your feedback!
Honestly that's probably not going to be a problem, or it's worth the change. If you really want to, you can invalidate sentences with "not" in them and you probably have all the realistic cases covered. Design for players that want to play the game, not for players looking to "hack" a game about pretty flowers :P