The plant that instantly crushes a single zombie when stepped on is Squash.
Squash is a single-use defensive plant in the original Plants vs. Zombies. It waits until a zombie comes within range, then springs upward and slams down, crushing the target before disappearing. Its name is a pun on both the vegetable and its attack.
Squash is often confused with Potato Mine because both are inexpensive, trap-like plants that remain inactive until zombies approach. Potato Mine must arm itself before exploding, while Squash is ready to react immediately. Jalapeno also destroys zombies, but it burns an entire lane rather than crushing one nearby target.
The original game, released by PopCap Games in 2009, divides the lawn into lanes and gives each plant a specialized role. Squash is especially useful when a dangerous zombie has reached a plant line and there is no time to build a larger defense. Like other single-use plants, it occupies a square only until its attack is triggered.