That... was unexpected.
Did you make the hull out of dirt? Maybe the game's growth algorithm grew trees out of dirt. (It's entirely possible that the game replaced the hull with dirt blocks. :D )
If it's made out of metal, I'm flabbergasted. It's like that one scene from "Creepshow", where Steven King grows alien plantgrowth all over his hands and face. (Safety tip, kids: don't handle meteorites yourself.)