They're poisonous plants, just like everything else in this garden so far.

[ She makes her way over to him, taking care not to touch any of said leaves herself, but very much wishing she'd worn longer pants in spite of the warmth. The ones she's got on end several inches above her ankles, and are loose enough that they might catch on something if she's not careful. So much for a casual walk to enjoy the flora.

She eyes the plant in question, mentally flipping through some pages of a book in her head. ]

I think that one's called hogweed? It can cause burning and blistering if you touch it.