Grave Buster

Usage
Simply plant on graves to remove them. Be careful however, zombies can eat the Grave Buster while it's working. You will also be rewarded with a silver coin (or, rarely, a gold coin, a diamond, chocolate, or a zen garden plant) once the grave is gone.

Strategy
There's not much to it: remove the graves you want to get rid of. Getting rid of the leftmost ones first is a good idea, as is evening out the number of graves in the rows. Removing graves from rows you aren't defending well is smart too.

You might want to get rid of all the graves when you have sun to spare, and try to do it before the final wave.