Seedum is a plant that sort of looks like broccoli flower heads, comes in several varieties and makes excellent tree foliage. The stems are really too thin and sparse to use as is, so most people bundle the stems together or use another technique. You can also lay the seedum on its side to mimic the smaller trees and bushes that make up the undergrowth in a forest.
Here's some seedum drying in my barn:
Painting assembly line. I usually spray the trunks grey and the foliage various shades of green. While the paint is still wet I sprinkle on ground foam:
Here's a shot of the finished product:
And on the layout:
