Vaulting Zombies can jump over it, but they cannot jump over a Tall-nut, its tougher counterpart. It is named after the edible seed walnut, specifically the Juglans regia. It is used to stall zombies to waste their time, letting other plants attack them. For more information of the Wall-nut please click here.
You can download this papercraft toy here: Plants vs. Zombies - Wall-nut Free Paper Toy Download