This combo is a reference to the Popcap game "Plants vs. Zombies."
"Plants vs. Zombies" is a tower defense game where you defend your house against hordes of zombies by filling your lawn with plants that can fight back.
If you wanted to put plants in your yard, what would you need to do so?
Where or what would these plants come from?
You would need some seeds to grow the plants.
To complete the combo, add a zombie.