Realistic looking scenery is important on model train set layouts. To model weeds, hay, straw, and grass get some “binder twine” from a farmer/horse owner. The twine is a natural (unlike the new plastic stuff) material that takes paint/stain well. It also weathers naturally. Hang some outside for a year and it will look like fall weeds, let it lay on the ground and it will get a grayish color. The twine can be cut in very short lengths and used as some ground cover.