This is never explained
The implication though is that the Predators have been in contact with Earth for millions of years.
There is a comment in the script:
They have been extinct since the early Cretaceous period. 125 million years ago.
Humans did not exist 125 million years ago so it's unlikely that the transportation of plants then would have been related to hunting them.
It's possible that the plants were transplanted to make the hunting of other creatures more interesting but there is nothing written about that.