There's something that might be considered an exploit. The insta-grow fertilizer can be used to bypass the Rocket Plant's cooldown, making it fire it's rocket instantly. This can be used to make the rocket fire over and over again for $400 per missile. It's still more cost effective to use regular fertilizer to get more rockets overall, but it can be use to trivialize enemies that rush in between the vulnerable spot when all of your rocket plants have fired and are recharging.