This answer first appeared on Quora December 26, 2019.
Could it be Greedy Guts (2000)? The original title of the film is Otesánek. It’s a Czech film, sort of a comedic horror.
A woman learns she cannot have children. Her husband, in an effort to comfort her, takes a tree root and makes it into a child.
The woman is rather unhinged and pretends it’s real. So much so, the root comes to life and needs a great deal of fuel in order to keep growing. That fuel happens to be people.
Image courtesy of flickr.