When giant, man-eating plants land on the moon, a pacifist gardener must protect his moon colony.

Haha, that's a great idea!

Love the idea! By adding "pacifist" you manages do add both an obstacle to the main character and give a hint of the theme. A question when pops to mind is how the gardener is going to protect the colony, and why he is the one that must do the protecting. Since the "monsters" are plants and he's a gardener, wouldn't it be interesting if he grew them instead of them landing? Maybe he tried to create a new plant that would save the colony from starving, but instead he messes up and the plants end up eating the colony instead. Just an idea.