Pumpkin

The Pumpkin is the thrid defensive structure in the game. Unlike the Wall-nut or Tall-nut, it does not take occupy a garden space for itself, allowing other plants [besides another Pumpkin] to be planted and protected in the same spot.

Usage
It costs 125 Sun to plant a pumpkin. Pumpkins can be planted on bare soil or another plant. Other Plants can also be placed in an empty Pumpkin. When a Zombie is attacking the space, the pumpkin takes damage until it is destroyed.

[Speculation] Pumpkins have the same health and profile as the Wall-nut

Strategy
Pumpkins are excellent on later levels, where the player needs to eke out as much space as possible. It is not unusual [although inefficient] to use Pumpkins to protect Sunflowers against Catapults.

When Bungee Zombies are about to attack a square, they take the plant inside the pumpkin first.