I agree with the use of Woodland SCenics trees, either ready made (a little thick on the foliage) or the kits. Some of the best diorama stuff still seems to come from model railroad scenery (IMHO), so look at these sections in hobbyshops, or get a look at the catalogs from the Wm K. Walthers Company. I buy the H-O one about every year (also because I like to model the Union Pacific RR). Lots of good "cross over" materials. Speaking of model railroading - you can look in some of the model railroad books and magazines for tips on making your own trees from all kinds of materials.
Another approach is to keep an eye of the Department 56 stuff and the various other lines of "village" materials. They often have some nice trees. One nice thing is that a number of stores stock these lines for the Christmas season and put them on 50% off sale as the holiday nears. Remember to look at what the tree "could be" rather than "is".
Except for the hedgerows by Conte, I'm not aware of other sources yet. Again, model railroad scenery would be the best way to get the tangled, overgrown embankment look.