Looks like the zombies gave up attacking your front yard. Now they are trying your backyard. And to top it all off, you can't even use your mushrooms! 'cause they fall asleep during the day! Well isn't that just dandy!
Plant Sunflowers first. Only worry about Lily Pads when the first Ducky Tube Zombie appears. For the first and second zombie that comes on the screen, plant Potato Mines to easily get rid of them.
Plant Peashooters and Repeaters for defense. Start planting Lily Pads and plant there also.
Plant Wall-nuts and Cherry Bombs if necessary.
Plant Snow Peas only when you are done planting Repeaters and Peashooters.