Tap a seed card at the bottom, then tap a lawn tile to plant it. Plants cost sun. Tap the suns that fall from the sky or sprout from Sunblooms to collect them, or turn on Auto-collect sun in the menu and they fly to you by themselves.
Zombies shamble down the columns toward your house and eat anything in their way. Shooters only hit zombies in their own column.
Each column has a garden gnome that clears it once if a zombie slips through. After that, the next one gets in.
Survive every wave to win. New plants unlock as you clear levels. Keys 1-9 and 0 pick seeds, X picks the shovel.