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 walk 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, with one of its large spikes to breaking off with every three vehicles. 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 and fast attacks a zombies.

Note: Spikerocks cannot be planted on Lily Pad or Flower Pot, like Spikeweed.

Suburban Almanac Entry
Spikerock

Spikerocks 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 (+100 if survival endless)

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, you can also use the Kernel-pult as if it uses a butter and a zombie is standing on a Spikerock, it will continue to take damage.

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

 * Spikeweed, Spikerock, and Lily Pad are the only flat plants in the game.
 * Jack-in-the-Box Zombies, Zombonis, Catapult Zombies, Squash Zombies, Jalapeno 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 without hurting them.
 * 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 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 or Jalapeno Zombie explosion to destroy a healthy Spikerock.
 * Biologically, the Spikerock is not a plant; it is a rock.
 * The Spikerock can still harm Bungee Zombies even if they are in the air.
 * The Spikerock is one of four plants that have names which don't involve plants, the others being the Grave Buster, Gold Magnet and the Chomper.
 * This is one of only two 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 Catapult Zombie's catapult machine explode by just popping the tires.
 * The Spikerock and the Squash are the only plants that can survive a Gargantuar smash and a vehicle.
 * The Spikerock is the only upgrade plant that can't be eaten.
 * The Spikerock, Spikeweed, and all Instant Kill plants, except for Chomper and Cob Cannon, are the only plants that can't be eaten.
 * On the iPod version, spikes will not fall off Spikerocks.
 * Occasionally, Spikerock can deal 9 normal damage shots or 11 normal damage shots in the 5 seconds the zombies are on the Spikerock.
 * The Spikeweed, Spikerock, Tangle Kelp, Sea-shroom, Potato Mine and Flower Pot are the only plants that cannot be planted on a Lily Pad.
 * The Spikeweed, Spikerock, Tangle Kelp, Sea-shroom, Lily Pad, and Cattail are the only plants that cannot be planted on a Flower Pot.
 * In Bobsled Bonanza, to kill Bobsled Teams, use a Spikerock plus a spikeweed behind it.