It gets trickier in the water.
The zombie war becomes a bit more trickier when the zombies decide to be sneaky and try to enter your house through the backyard instead. You actually have a little pond back there so you get to work with a fine bunch of aquatic plants to keep the zombies out. Here's a little piece of advice: the Tangle Kelp is really helpful in bringing down the snorkeling zombies and the Squash is really good for those instant kills, so it would be wise to give those two plants some permanent space in your seed slots.
The fog is on their side.
Eventually night time rolls around once again but this time you find your backyard only partially visible amidst a dense and heavy fog. You still have those trusty shrooms to rely on but you've got some new nocturnal additions like the valiant Sea-shroom that puffs away at your unseen enemies in the pool. You could also take advantage of these two plants so you can see what you're up against better: the Blover, which will blow away the fog to clear the air for a short period of time, or the Plantern, which will provide some decent lighting in its immediate surroundings.
The roof becomes a battefield.
The zombies try to enter your house from the roof as a last resort. Things start to get complicated because the inclination of the roof makes hitting the zombies a lot harder. Your Peashooter's peas can't get to them anymore, so it's time to pelt those suckers with cabbage bombs and corn kernels with the occasional sticks of butter care of the Cabbage-pult and the Kernel-put.
When the zombies realize that they can't defeat you on their own, they call on their big boss, Dr. Edgar George Zomboss, to help them defeat your green army. I'd tell you what the game's final boss is capable of doing, but where's the fun in letting you know way ahead of time?