Flamethrower

A flamethrower is a device used to launch fire at a target. When Agent Len went insane, he tore down the corridors with one; he sought to kill Mr. Rogers.

Understandably, due to their physical nature, the Flowers That Be do not like Flamethrowers. It's possible, though not known for sure, that flamethrowers are the only weapons that actually can kill a Flower, considering that it seems to be the usual means of dispatching one.

Additional Information
Flamethrower on Wikipedia.