When in a difficult situation, you may be forced to dig up plants. Removing a useless plant from a space early instead of waiting for a zombie to eat it can give you time to replace it with a more useful plant, then eliminate the zombie before it gets too close. Digging up plants also restores some of their sun cost, which may increase it to the point for you to set down a important plant. This strategy is very useful when there is one lane of the field that is being focused by zombies while the other lanes are completely open.
Plant food
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.
Plants
Keep a variety of short and long range plants ready. For example, have long-range plants such as Peashooters and Cabbage-pults along with close-range plants like Bonk Choys and Snapdragons. Plants such as Iceberg Lettuce, Walnuts, and Kernel-pults that slow down approaching zombies are very useful. They will keep zombies within range of your long-range plants to soften them up for your close-range plants. Start by placing a Sunflower. As soon as you have the resources, purchase a second one. Always have two full columns of Sunflowers planted, and you will not have to worry about the sun. Consider placing them in the first column and the third column. By doing this, zombies in more challenging levels will destroy those Sunflowers and not your offensive plants. You can then replace them with offensive plants as needed. Place Pumpkins to protect your plants. Try to have a complete line of Pumpkins to block zombies. They will slow down the game enough for you to have a chance to develop your strategy. Place the Pumpkins as close to the starting line of the zombies as possible. During survival levels, work quickly. Place offensive plants on the row that requires them the most. Defensive plants should be place in rows that have no offense yet. Be ready to quickly use each plant and move it where it is needed the most. Using Potato Mines will give your more time by blowing away some enemies. It costs twenty five Sun and is very useful. It has a fast recharge rate and can work very well as a last line of defense against a zombie. The Bloomerang is also very useful. Use it in Ancient Egypt for the series of zombies that are hidden behind obstacles. The Bloomerang attacks the entire row twice, one when it launches the boomerang, and once when it returns. It is especially effective when zombies appear in series. Do not place a Pea Shooter in a lane that is filled with tombstones. Instead, use plants that can throw over obstacles. The Bloomerang is a good choice in this situation, because it hits everything in its way twice.
Power-ups
Save power-ups until you absolutely have to use them, usually in special missions or star missions. There are three different power-ups, with the "Throw Away" being most useful. Do not spend too much on power-ups. They are powerful but also expensive and not always required. Use power-ups during star levels due to their difficulty.
Save the day with a Banana Launcher
This has got to be my favourite plant as it allows you to target a specific zombie in any lane, in front or behind the plant and smashes it usually in one hit. It's expensive at five hundred sun so it may be a while in the level before you can plant one.
Sun-shroom
This mightly little mushroom grows after you plant it to produce more and more sun. It's super cheap at only twenty five sun to plant and if you can manage to plant a head of these (usually in the first column) and protect them they will grow into awesome sun producers.
Wall Nut and Bonk Choy
These two plants make a fantastic combination and defeat so many zombies really easily. Basically you just need to plant a bonk choy directly behind a wall nut (or similar blocker such as a tall-nut, infi-nut or endurian) and it should be an effective smasher of numerous zombies. You can also team this combo with spikerock or spikeweed in front for even deadlier line-up.