The item that allows plants to be planted on the roof is Flower Pot.
Flower Pot provides a soil-like space on the sloped roof in the original Plants vs. Zombies. Because the roof is not ordinary ground, players must place a Flower Pot before planting most ground-based plants there. The pot is therefore a support item rather than an attacking plant: it makes the roof’s unusual terrain usable.
The roof is the fifth and final stage of the game’s Adventure mode. Its upward slope also changes the way many attacks work, encouraging the use of catapult plants, whose shots arc over the incline. Flower Pot solves the planting problem, while the roof’s geometry creates a separate tactical challenge.
It is often mixed up with Lily Pad, which provides a platform for plants in the backyard pool, or Plantern, which clears fog around itself. Pumpkin protects a plant by forming a defensive shell but does not create a place to plant. The roof therefore has its own terrain-specific item, just as the pool has Lily Pad.