What plant acts as a wall to block zombies?

The plant that acts as a wall to block zombies is the Wall-nut. It has no offensive attack, but its high durability lets it stand in a zombie's path and buy time for attacking plants to do their work.

Wall-nut is one of the original game's most important defensive plants. Players commonly place it in front of Peashooters and other attackers so zombies spend time chewing through the nut instead of reaching the vulnerable plants behind it. This simple combination teaches the game's basic division of labor: some plants generate resources, some deal damage, and others absorb attacks.

Tall-nut is a stronger related defensive plant that can block certain jumping or flying movements, so it can seem like an equally plausible answer. Pumpkin can also protect a plant by enclosing it, but it is not the game's standard freestanding wall. Spikeweed damages zombies that walk over it, while Wall-nut's defining purpose is to block and delay them.