I just came across a new product, I found it at my local Michael's craft store. They are molded plastic "blocks" of vegetation. I got two, there are about 5 or 6 different kinds of plants that you can get. The blocks are about 6" square, and each holds about 25-30 plants, and the blocks sell for about $6. I got one leafy bush kinda thing, and one more like rushes or grass. You just clip each plant off the sprue, and I stuck them in a little hole in the layout with some glue.
They are shiny plastic, so for some of you they might not be realistic enough. They look great on my tinplate layout, and I thought at about 20 cents a bush it was a decent price.
