I have a blog about SketchUp and landscape design called http://sketchupland.posterous.com/.
It's been more general stuff lately, but I'll have more landscape specific articles soon.
Here is a list of good 3D trees from 3D Warehouse: http://sketchupland.posterous.com/best-3d-trees-from-3d-warehouse.
There are a number of good landscape designers/architects on the forum. Hopefully they will showoff their inspiring work, and show you the ropes.
http://www.verdantdesignky.com/wp-content/uploads/2010/12/1.jpg