Here's a (relatively) new theory: in the Lunaverse, the word "weed" is actually(/also) a Moonflower slur. It means "useless, worthless" and is used to refer to "defective" Moonflowers (i. e. ones whose moons have been struck by the Big Bad). I'm not really sure if Hanazuki would've known about this.

I'm also thinking that Twisted has called Kiyoshi a "weed" in the past, and now Kiyoshi can't stand to hear that word used (especially when used to describe any of his friends). When describing his past self, he prefers to use the term "Moon-failure."

Maybe there could be a villain who believes that the only way to ultimately destroy the Big Bad is with a "weed sacrifice" (having the Big Bad take the life of a failed Moonflower).