Potato Mines take 15 seconds before they arm themselves (approximately the time it takes most zombies to walk three tiles). Some zombies may eat it or jump over it, so you should not place it directly in front of a zombie. Another easy mini-game to get this on is Column Like You See 'Em. Just plant the Potato Mine right before a huge wave of zombies appears. If you need some extra time to let the Potato Mine arm itself, place a defensive plant, such as a Wall-nut in front of it, then shovel up the defensive plant when the Potato Mine has been armed.