Thank you. It's functions, no image maps for placement. Search the grid function I posted somewhere here.
Yes sometimes objects populate slightly outside the boundaries, so you just need to make another mask that is smaller than the area the plants are supposed to be in. If you give it a color higher than 1, like 3, you give the populator a very high probability to place an instance exactly there. However you also need to use a relatively small object distance in the populator to ensure the populator sampling "hits" your white mask spot.
Cheers,
Frank