Pirah-nut

Pirah-nut is an aquatic plant. It absorbs 30 bites. It also bites back at zombies that are trying to eat it, dealing 1 damage every second. However, if there are other Pirah-nuts onscreen, they will quickly swim over to a Pirah-nut that is being eaten, and help bite the zombie. They do not provide extra health for the Pirah-nut, just extra damage. If the Pirah-nut is fully eaten, the others will swim back to their own tiles.

Strategy
One Pirah-nut by itself is not fully effective. Try planting a full row of them.

Weakness
If Pirah-nuts swim over to help another, zombies can get by their tile.