It freezes all zombies in a large area around it, but it attacks very slowly. That makes it very vulnerable between blasts.
Its weapon is called Ice Blast, and each blast deals 20 to 25 damage based on distance.
CLOSE RANGE: ICE
Lets out bursts of icy-coldness, freezing nearby Zombies where they stand.
The Snow-shroom is the Ice-shroom's long lost cousin from the North Pole.
Ice-shrooms do not stand much of a chance vanquishing zombies themselves, so it is best that you stay near them, so if a zombie gets frozen you can easily vanquish it. They are not capable of freezing Yeti Zombies, Giga Gargantuars, or Gargantuars, so they are useless against them.
However, they can freeze Baron von Bats and Disco Zombies. In Gardens & Graveyards and in Garden Ops, they should be planted near the garden. This way, if a zombie is fighting a plant, they will probably too distracted to notice the Ice-shroom and will get frozen, making for an easy vanquish. You should also plant them near the taco stand in Taco Bandits, but, however, the plant pots are not exactly close to the stand, so try to find the nearest one.
Plants vs. Zombies Wiki has a gallery for Ice-shroom.