Some Artistic Help Needed

Wisps of shrubbery, a hint of a tree or two, and other greenery. Doesn't even have to be very specific--you just have to get rid of the "emptiness" in a plausible way. Expanded strands of that fibrous "flock" stuff that the scenery suppliers sell is good for this kind of thing. 

Past the sides of the houses, I would make a flat out of foam core with glued-on ground foam, etc, and blend it into the background image. The top should be an irregular border of tiny tree-like shapes, not a straight line.

If you didn't want to go to a lot of work, just painting the backside flat black or charcoal would probably provide sufficient shadowing effect. Or painting cardstock if you didn't want to disturb the wall.

Match the tree color in the backdrop. Have the paint computer analyze and mix a small batch of paint,  in Home depot.  Then weather the row houses to set them back into the scenery.  Have fun!

What Patrick said,  Try to match the foliage color of the backdrop.  It would give the illusion of two streets separated by a natural buffer.  Since the houses on the backdrop raise in elevation from right to left I would try to add a slight elevation change in your street from left to right.

coach joe posted:

What Patrick said,  Try to match the foliage color of the backdrop.  It would give the illusion of two streets separated by a natural buffer.  Since the houses on the backdrop raise in elevation from right to left I would try to add a slight elevation change in your street from left to right.

Funny you mention the elevation Coach Joe.  That area actually does go up from right to left and I did plan on making the structures follow the terrain elevation.

Good eye, Ron

A line of trees.

Even another somewhat tone matching backdrop of trees with no sky. You might trim the tree top area to loose the straight cut line and hide the tree top horizon. Furs would take a lot of jagged cutting, but full round trees should result in smoother "cloud shaped" horizon line that blends well to the top mural without devolping carpel tunnel..

  A darker tone will highlight your building and top mural more, and indicate the trees are in a down slopes shade or shade of another hill....I think? It looks like the sun is behind us & on the right; but you can see it better (?). Far right sun would put dark trees sort of between the sun and the color homes.

   If you add a drops or use them to fill you should note that the suns position should likely match more than the tone! With effort, you can employ diffent distance perspectives with different shadow positioning.

  I.e. two shots with different sun angles, #1 on the left and#2 on the right may not look as correct as #2 on the left and #1 on the right.

Or

An ornate, and/or ivy coated brick retaining wall. I think I'd match at least part of it to the porches color if your really wanting to tie them together as a newly unified group as strongly as possible. If the wall predates the houses, red/orange or grey cinder for a poorer area; yellowish tan, stone or deep red brick for a nicer one. (That is debatable, but how I see it anyhow)