Flamethrower

"The very existence of flamethrowers proves that some time, somewhere, someone said to themselves, 'You know, I want to set those people over there on fire, but I'm just not close enough to get the job done.'" -George Carlin

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

Understandably, due to their physiology, the Flowers do not like flamethrowers. While not the only way to dispatch most Flowers*, 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 it's dead.

* 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&mdash;shooting a tree with a pistol or stabbing it with a sword doesn't tend to be very effective.

Additional Information
Flamethrower on Wikipedia.