From Plants vs. Zombies Guide
Snow Peas are the second shooters you get in the game, and one of the most useful. Like Peashooters, Snow Peas shoot peas at zombies with an unlimited range, but they shoot frozen peas which both damage and slow zombies down. This means Snow Peas essentially add one damage per second and double the attack power of the lanes they're planted on.
After building the minimal setup consists of Peashooters and Sunflowers, start planting Snow Peas so that each lane contains one Snow Pea. After that, plant other shooters in front to further increase the firepower. You must note that the freezing power of Snow Peas don't stack up, so plant only one each line. With Snow Pea in the build, your defense would become 2x stronger.
NEVER use Snow Peas with Torchwoods, because Torchwoods thaw the frozen peas, turning them into normal peas. Placing them in front of Torchwoods won't work either, since the flaming peas from behind will eliminate Snow Peas' freezing effect.
Snow Peas shoot frozen peas that damage and slow the enemy.
Damage: normal, slows zombies
Folks often tell Snow Pea how "cool" he is, or exhort him to "chill out." They tell him to "stay frosty." Snow Pea just rolls his eyes. He's heard 'em all.