What plant freezes zombies when it hits them?

The plant that freezes zombies when it hits them is Snow Pea. In Plants vs. Zombies, Snow Pea fires icy peas that damage zombies and slow their movement, giving other plants more time to attack.

Snow Pea is one of the original game's daytime plants and costs 175 sun to plant. Its projectile deals the same basic damage as a regular Peashooter’s pea, but adds a chilling effect. The slow applies to most ordinary zombies, although special mechanics and some later-game variants can change how effective it is.

A common mix-up is Winter Melon. Winter Melon also uses cold-based attacks and slows groups of zombies, but it is an upgraded lobbed-projectile plant rather than the basic single-target freezer. Iceberg Lettuce, introduced in Plants vs. Zombies 2, freezes a zombie through a direct contact effect instead of repeatedly firing frozen peas. Snow Pea’s distinctive identity is its steady stream of icy projectiles.