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. While not the only way to dispatch most Flowers (though some, such as the Bracket Fungus or Hornbeam, are heavily resistant to other weapons just due to their physiology; the Fungus due to his soft hide allowing strikes to pass through without damage and the Hornbeam because he is a tree, and thus wouldn't be severely damaged by most weapons- shooting a tree with a pistol or stabbing it with a sword doesn't tend to be very effective), as blades and bullets have been shown to work just as well, they seem to be the preferred anti-Flower weapon by far. This might be because burning a Flower is a much easier way to make sure they're dead.

Additional Information
Flamethrower on Wikipedia.