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, as though hit by a frozen pea or frozen melon. It is unlocked after level 2-7, and it is the 15th plant you unlock while going through your adventure.

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 them in 5.5-second intervals to maximize freezing time, 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 or crush 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 achievement, 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 a good option.

Trivia

 * It is the only mushroom and plant that can freeze zombies and one of only four plants that can slow zombies. The other plants able to slow zombies are the Snow Pea, the Winter Melon, and the Iceberg Lettuce.
 * Its power is similar to Iceberg Lettuce's Plant Food ability with a slowing effect
 * It is the only freezing plant in Plants vs. Zombies that does not reappear in Plants vs. Zombies 2: It's About Time.
 * 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.
 * It might have been injured, as its face is always in a frown, and his Almanac entry states that it was in an accident.
 * 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 be frozen or slowed 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 an instant kill. It does do damage to all zombies on screen, but that damage is the equivalent of a single pea.
 * Ice-shroom and Potato Mine are the only explosives that do not burn zombies into ashes.
 * When used on a Pool or Fog level, the pool's animation stops until the zombies are unfrozen.
 * The Ice-shroom, the Grave Buster, and the Umbrella Leaf are the only plants that can prevent Surprise Attacks from occuring.