At the hobby store they sell different bits of terrain. The stuff below is used for fields, or wild flowers, or tree canopies.
It is long and stringy like cotton. Dab your glue and water mixture on the landscape and press a small tuft of this stuff in place. Shape and manipulate with a toothpick or sculpting tool.
It can be pulled really thin and sparse or left in heavy clumps.
That is how it can look very sparsely placed.
The color breaks up the patches of green. You can find lots of different colors. The more you use the more realistic it can look if you do it correctly.
In your mind, figure out what kind of plant you want this to simulate. Decide what kind of growing conditions it would need. For instance, let’s assume this is a yellow plant that needs full sun and some water to survive. Now you can look around your set for places that these conditions exist and place your plants there.
As you plant your landscape, imagine where water would collect, where runoff would come down the mountain, where seeds would collect, etc. It doesn’t matter what rules you follow, just be consistent throughout your project.
The whole goal here is to convince the brain that this is real somewhere and not painted styrofoam. The mind starts off filled with disbelief that it is real. We are trying to “suspend the disbelief”. We are trying to convince the viewer’s brain to let go of some of that disbelief.
We can see a whole scene and we can believe it is real. But if we notice something that is not right, or doesn’t follow the rules, then we immediately fill our brains with disbelief, and the illusion is ruined. For example, if the entire set is filled with yellow plants that live in the sunny areas, then in one spot we find them in dark shade, that might be enough to cause us to not believe any more.
You won’t ever get it perfect, but if you make some rules, and stick with them you will have a much better chance to fool your viewers.
Go ahead, plant some sunny yellow flowers, make a mess, have some fun.