Trees and grass

I'd say you make a grayscale grass map (there was a recent forum post about that at http://www.blitzbasic.com/Community/posts.php?topic=35433) and create the vertices for the grass (I would highly recomend using a single surface system for something like this). You would use TerrainHeight(terrain,x,z) to find the bottom vertex's Y position.

[edit]You could still use a variation of this idea, but unless you are totally insane or have an amazing occlusion system planned out, I highly recommend using sprites for this. [/edit]