Spikerock

Spikerock is the upgrade of Spikeweed, and is purchased from Crazy Dave's shop for $7,500. It is a passive attacking plant, dealing high damage to any zombies that tread on it. It costs 125 Sun to upgrade a Spikeweed to a Spikerock, for a total cost of 225 Sun. It usually deals ten attacks in groups of two (each attack doing one damage) for every normal speed zombie that passes unimpeded over it (exceptions include zombies that come in a group, when one zombie gets on just after the Spikerock fires). Spikerocks will pop tires on up to nine Vehicles (Zombonis and Catapult Zombies) before death, every three vehicles causing one of its large spikes to break off. When a Gargantuar attacks it, it will have the same effect as when vehicles roll over it, without the Gargantuar dying. Since the Spikerock attacks from below, it bypasses shields.

Suburban Almanac Entry
Spikerock

Spikerock pop multiple tires and damage zombies that walk over it

Must be planted on Spikeweeds

Spikerock just got back from a trip to Europe. He had a great time, met some wonderful people, really broadened his horizons. He never knew they made museums so big, or put so many paintings in them. That was a big surprise for him.

Cost: 125

Recharge: very slow

Strategy
Use the Snow Pea, Winter Melon or Ice-shroom to slow the zombies, so they spend more time being attacked by the Spikerock. A Snow Pea and a Spikerock is particularly cost effective, but if Winter Melons are used, all zombies get slowed.

Plant the Spikerock in front of a Wall-nut, Tall-nut, or Pumpkin. Most zombies do not attack it, and are continuously damaged as they try to break down the defensive plant.

The Spikerock is (uniquely) capable of withstanding multiple strikes from Gargantuars. This considerably slows Gargantuars and allows extra time for your other defenses to attack, since, unlike other zombies, Gargantuars will stop to destroy the Spikerocks.

Bobsled Bonanza Mini-game
As it can pop multiple tires, the Spikerock is an incredibly useful plant to use on this level. It is a good idea to plant a column of Spikeweeds at the edge of the ice to deal with Zombonis, but they should be upgraded to Spikerocks as soon as possible. To deal with the Zombie Bobsled Teams, there should be an additional Spikeweed behind the Spikerock (as the zombies can only take one normal damage shot after passing over the Spikerock, although when first setting up it is best to plant three).

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.
 * When a Spikerock is planted in one of the two bottom right squares, it will be higher up than other Spikerocks. This is so you can see them over the level progress bar.
 * The Spikerock is the most durable and toughest plant in the game because it can withstand 9 Gargantuar smashes and 9 tire pops of the Zomboni and Catapult Zombie.
 * Despite its resilience, it still only takes take one Jack-In-The-Box-Zombie explosion to destroy a fully healthy Spikerock.
 * Biologically, the Spikerock is not a plant; instead, it is a rock.
 * The Spikerock can still harm Bungee Zombie even if they are in the air.
 * The Spikerock is one of the four plants that have names which doesn't involve plants, the others being the Grave Buster, the Gold Magnet, and the Chomper.
 * This is one of the plants that change appearance, but cannot be healed with Wall-nut First Aid. The other being the Garlic.
 * It is unknown how the Spikerock can make the Zomboni's ice machine and the Catapult Zombie's catapult machine explode by just popping the tires. Probably because when Zomboni's ice machine tires popped the Zomboni cannot be used.
 * The Spikerock and the Squash are the only plants that can survive a Gargantuar smash.
 * The Spikerock is the only plant upgrade that can't be eaten.
 * It is an upgrade that is a little more expensive than the original, the other is the Gatling Pea.
 * The Spikerock and the Spikeweed are the only plants that can't be eaten.