The upgrade to Melon-pult that also slows zombies is Winter Melon.
Winter Melon is the cold-weather upgrade of Melon-pult in the original Plants vs. Zombies. It lobs frozen melons that strike a target, deal splash damage to nearby zombies, and apply a slowing effect. This combines Melon-pult’s heavy area damage with crowd control.
Players sometimes choose Melon-pult itself because it is the base plant, but it does not provide the freezing slowdown. Kernel-pult can stun zombies with butter, and Cabbage-pult launches projectiles over obstacles, yet neither is the direct upgrade described here. Winter Melon requires a Melon-pult already planted on the lawn before the upgrade can be applied.
The distinction matters most in late-game waves, when slowing a group gives other plants more time to fire. Its lobbed attacks are also useful against defensive zombies because catapult projectiles can pass over certain shields. Winter Melon is expensive in seeds and sun, but its combination of damage, splash coverage, and slowing makes it one of the original game’s defining support attackers.