You're a little off on which plants need which fertilizer.
White-labeled plants are commons and IME never need fertilizer. "Uncommon" (blue labeled) need Tier 2. "Rare" (magenta labeled) need Tier 3.
I consider that spoiler information, BTW, but since you've posted it I'll correct it. I will leave the purpose of tier 4 fertilizer as an exercise for the reader.
I didn't know that about the highlights in the store. I'm excited to look at that today; I know one of my games has a brown seedling so it should be a hybrid.