Used in the sentence:
There was no garden when they moved in, so they are not (???) any existing plants.
I wanted to say threatening, but I am curious if a word specifically relating to this phenomenon.
How about “competing”? As in, “They are not competing with any existing plants.”
The word “competition” comes up quite frequently in the study of biology and ecology when describing the interactions between two or more species, hence my suggestion.
"threatening" is good, or as lightweaver said "competing with", crowding, hindering, affecting, incompatible with, shading.
i assume you are introducing new plants to an existing space
...they are not (???) any preexisting plants.