When you get access to the limited-use plant food that allows you to activate a specific plant's super attack, save it until absolutely needed. When used at critical moments, the plant food can help you win a difficult situation. Wait until there is a glowing green zombie, which always rewards one use of plant food when killed, appears in the middle of a pack of other zombies. Use plant food to take out that group easily, and immediately get plant food back from the kill. Do not start with more than one plant food due to its cost, and the fact that you should get a few during a mission. Use them all before the mission ends, as they will disappear.