I have to agree with Aegis159,
Foam is the way to go now adays. I work for a model
railroad manufacturer as a kit designer and we use
the pink 2" (or 1") insulation foam available at most
home centers (Home Depot or Lowes).
Woodland scenics makes most of the products you would
need to a landscape layout. I also found that using
dried floral and doing some light airbrushing gives you
more realistic looking trees and brush that the items that
they carry. For me, Trees and shrubs are the toughest
things to get to look real.
I guess it all depends on what you are planning on modeling.
Hope that helps you some.