A strategy to make a ton of cash fast and easy.
This strategy is a simple, streamlined money farm right in your own front lawn! This can be attempted after earning the marigold on level 5-7. It requires only four plants: sunflower, marigold, garlic, and any attacking plant.
Start by placing a column of sunflowers in the back, and placing marigolds as they recharge in rows 1, 3, and 5. At the front lines, place garlic on rows 1, 3, and 5 to put all zombies in rows 2 and 4, which is where the attacking plants reside. Your lawn should look like this:
You can earn a few thousand coins per level with this strategy, and all the upkeep you'll need is just replacing garlic. I have added here a double layer of garlic to prevent any pole-vaulting zombies from getting past, and some chompers as a little insurance.