Spikeweed

The Spikeweed is a passive attacking plant, dealing normal damage to any zombie that walks on it.They can only be planted on the ground ( not even Lily Pads or Flower Pots) and does 5 attacks in the time it takes any normal speed zombie to pass over it. It will also pop the tires of the first Zomboni or Catapult Zombie that drives over. Also, since the Spikeweed attacks from below, it can bypass shields.

Suburban Almanac Entry
Spikeweed

Spikeweeds pop tires and hurt any zombies that step on them.

Damage: normal

Range: all zombies that walk over it

Special: can't be eaten by zombies

Hockey is Spikeweed's obsession. He's got box seat season tickets. He keeps close track of his favourite players. And he consistently cleans up in the office hockey pool. Just one problem: he's terrified of pucks.

Cost: 100

Recharge: fast

Standard Levels
Plant Spikeweeds in front of Wall-nuts, Tall-nuts or Pumpkins. Most zombies can't harm it, and will be continuously damaged as they try to eat the defensive wall. Spikeweed is especially important for levels with Zombonis, and placing the Spikeweed on the first row will prevent the Zomboni from laying any ice at all (preventing the appearance of the Zombie Bobsled Team). Note that Gargantuars can destroy Spikeweeds in the same way as they can any other plant.

Vasebreaker: Hokey Pokey
Try to lay down either a row of three Spikeweeds or one Spikeweed in front of a Wall-nut to kill the zombies. Timing can be everything, so if you see a Wall-nut, go ahead and break a few vases on the right hand side to see if a Wall-nut will be needed there. If no zombies appear closer than the fourth row from the left, put the Wall-nut on a different row, and lay down Spikeweed instead. Buckethead Zombies are the exception to this, as you can't kill them with just Spikeweed - make sure to have a Wall-nut in place or use a Squash to kill them.

Trivia

 * Jack-in-the-Box Zombies, Zombonis, Catapult Zombies, Gargantuars and Giga-Gargantuars are the only zombies capable of damaging Spikeweeds and Spikerocks. This does not include Bungee Zombies, which can steal them but not hurt them; however, it appears that a Bungee Zombie must come down more than once to steal these.
 * Even though Spikeweeds can't be planted in Flower Pots during levels, Spikeweeds are always in Flower Pots in the Zen Garden.
 * This is probably because a Spikeweed would be useless in a Flower Pot, as zombies wouldn't walk over it, but they don't need to attack any zombies in the Zen Garden.
 * The Spikeweed is actually too wide for its Flower Pot, so it sticks out over the sides which may seem unaesthetic.
 * Spikeweeds also have a faster attack rate than other plants.
 * When a Spikeweed is planted in one of the two bottom right squares, it will be higher up than other Spikeweeds. This is so you can see them over the level progress bar. However, other reasons why this occurs are still unknown.
 * The Spikeweed seems to look like mud with spikes on top of it.
 * It is unknown how a Spikeweed can destroy Zombonis and Catapult Zombies just by popping their tires.
 * The Jack-in-the-Box Zombie, the Zomboni, the Catapult Zombie, the Bungee Zombie, the Gargantuar and the Giga-Gargantuar are the only zombies that can destroy Spikeweeds and Spikerocks.
 * Alone, it takes two Spikeweeds to destroy the regular Zombie.
 * When a Zomboni hits a Spikeweed and then a Squash tries to squash it, the Zomboni will not explode when the Squash squashes it.
 * Although Spikeweeds are known to give 5 damage to normal speed zombies, the zombies sometimes will get hit 4 times or 6 times.
 * Maybe because some zombies are faster and some zombies are slower.
 * It is unknown how the Spikeweed can make the Zomboni's ice machine or the Catapult Zombie's catapult machine explode by just popping the tires.
 * On the XBLA version, on Co-op mode you can butter the zombie and make the zombie stuck on the spikeweed.