The current icon is a reference to the phrase "green thumb," which means a natural skill for gardening. The hand on the new image only has four fingers and the thumb turns green, just like a regular zombie.
'Morticulturalist' is a combination of the word "horticulture," which means gardening, and the Latin root "morti," which means to have a relation to death or the emotions relating to it. In French, the translation of "Morticulturalist" is "Phytophile," which means "someone who has a passion for plants."
The lawn ornament in the Xbox Live Arcade version is a garden gnome, with Crazy Dave's face, holding a Peashooter.
Morticulturalist is the only achievement that Crazy Dave personally acknowledges. It is also the only achievement that does not play the achievement jingle when the player gets it.