Ice-shroom

The Ice-shroom is an instant use Plant. When planted, it will immobilize all Zombies on screen for 5.5 seconds. Once unfrozen, the Zombies will still be slowed down.

Suburban Almanac Entry
Ice-shroom

Ice-shrooms temporarily immobilize all zombies on the screen.

Damage: very light, immobilizes zombies

Range: all zombies on the screen

Usage: single use, instant

Sleeps during the day.

Ice-shroom frowns, not because he's unhappy or because he disapproves, but because of a childhood injury that left his facial nerves paralyzed.

Cost: 75

Recharge: very slow

Standard Levels
Plant the Ice-shroom when tough Zombies are trying to eat your other plants. Against Dr. Zomboss, the Ice-shroom can be used to destroy fireballs, and also to freeze him so he can be damaged for more seconds at a time.

Note: Try to not spam them in the boss fight, when he puts his head down use as much as you can, until you feel that he is going to go up.

In Survival: Endless, it is an extremely useful plant, especially in cobless set-ups, where the few seconds gained with the Ice-shroom matters. It may be a good idea to plant an Ice-shroom before it is needed so it can be saved for later, but this may not work if there are hard-to-stop Zombies that could get to it and eat it. They are normally placed in the Pool or near the back of the lawn, where Zombies pose less threat.

In the iOS versions of the game, freezing 20 full-sized Zombies with a single Ice-shroom will earn you the 20 Below Zero achievement. The Ice-shroom can be planted anywhere and still freeze all on-screen zombies. It can also be used during Invisi-ghoul to show you where the zombies (on the land) are.

Whack a Zombie Mini-game
Use the Ice-shroom when you need a little extra time to whack a large wave of zombies. If you have enough Sun, you can take that time to drop a Grave Buster at the same time. Use this only during the later part of this level as there will be more Zombies.

20 Below Zero
To get this acheivement you need to freeze 20 zombies with a single Ice-shroom. The two best places to do this are Last Stand and Survival: Endless, where the zombies come in big numbers. However, make sure to bring a Coffee Bean to wake the Ice-shroom up. Level 2-10 is also good option.

Trivia

 * It is the only mushroom and plant, that can freeze zombies and one of only three plants that can slow zombies. The others are the Snow Pea and the Winter Melon.
 * It is the only movement-restricting plant that does not use a projectile to restrict movement.
 * The Ice-shroom, the Doom-shroom (when exploding), the Fume-shroom and its upgrade are the biggest mushrooms.
 * The Ice-shroom and Blover are the only plants that have a known childhood story.
 * Ice-shroom has icicles like the Snow Pea and Winter Melon.
 * The Ice-shroom, Fume-shroom, Doom-shroom and Gloom-shroom are the only mushrooms that frown.
 * He might have been injured, as his face is always in a frown, and states that he was in an accident in the Almanac Entry.
 * When an Ice-shroom explodes, snowflakes can be seen.
 * Ice-shroom is one of the twelve plants that appear in the seed selection screen after you get the Grave Buster on the online version, the others being: Cactus, Lily Pad, Potato Mine, Threepeater, Tangle Kelp, Jalapeno, Spikeweed, Torchwood, Tall-nut, Doom-shroom and Blover.
 * There are three zombies that cannot freeze or slow down by Ice-shroom. They are Zomboni, Zombie Bobsled Team when they are in the Bobsled and Digger Zombie when he is underground.
 * It is the only explosive plant that is not a instant kill.
 * Ice-shroom and Potato Mine are the only explosives that do not burn zombies into ashes.