@Lucius CypherThat's a text-to-speech reference, isn't it?
@BananaIf you want powerful uses of plants, I have a couple of suggestions.
Airborn moss spores which rapidly grow when inhaled, filling up the lungs and internal organs of the victims while draining them of blood and power.
Trees that naturally produce nitroglycerine berries, and are capable of launching dozens of those berries by shaking their branches.
Mushrooms that explode into a cloud of acidic gas when anyone gets too close, dotted around the battlefield like mines.
You don't need to control the powers of others when you can make up your own. With something as general as 'control over plants' you can take any ability any real-life plant has, and then exaggerate it immensely, with the excuse that Hell Magic or Machina Tech made it extra-badass. And with real life plants like the Doki Doki tree there's plenty of stuff to draw on.
Oh and if Victoire's roses are annoying just create a grapevine-like plant that latches onto the flowers and drains them of blood. As they require blood to function this would cause them to rapidly wither. Pretty sure the same trick would work on Helena.