Question
Hi Marc,

I have a 75' retaining wall at the rear of my yard. I am going to dig the mulch out and replace with stone and plant new trees. Here is a picture of what the last owner left us (don't mind the homes behind, you get the idea, I stitched together a few pictures and it didn't do a great job).

http://fototime.com/198A3BE471C2A1B/orig.jpg

I may leave the plum trees on the ends but will remove the juniper and the dead bushes. The Plums are about 10' tall and growing nice. I want something that will grow at least 12-15' to block the homes behind us, but not get too tall that it makes the yard like a "prison." Also want something low maint. that looks nice (color would be nice). I was thinking of these two options:

A solid wall of Emerald Green Arborvitae. They would start at 6' tall and grow from there. How many trees would I need (spacing?)

or

Put a 6' solid vinyl fence up, and plant more plum and some pear trees on the wall (spacing??). Eventually they will make a solid tree wall above the fence and add some color in the spring. The only issue is that I don't want anything too wide because it will extend too far over the wall into the grassy area of our yard, which isn't that big.  Our property only goes 4' beyond the wall.

Looking for your opinion or possible a fresh idea

Answer
4 feet is plenty of space to work with behind the wall.  Here are some screening plants that would work in your situation:

Emerald green arborvitae ?space 3?apart, 25 total; Gets 15-20 ft. tall in about 10-15 years.

Thuja 慓reen Giant?(Thuja Plicata) space 4?5?apart, 15 to 18 total; Gets 20 ft. tall

Leyland Cypress ?space 4?apart, so 18 total; Gets 15-20 ft. tall

I would suggest something to spill over the front, as well, to soften the look of wall a bit.  Possibly Clematis, Silver Lace Vine, or Boston Ivy.  Avoid Virginia Creeper, as it goes more UP than down it its vining habit.

Regarding a fence & pear combo, you might consider an "espaliered" fruit tree (pear, apple, peach).  A trimming technique that flattens the tree out ?

Examples here:.  
http://www.frenchgardening.com/tech.html?pid=312615870324755
http://wherethefishlive.blogspot.com/2011/04/espaliered-apple-trees.html

It takes a while to train, it is a bit of work to maintain, but very unique when combined with a fence. I抎 plant them about 5-7 feet apart and they will grow together over time.  Fruit trees are deciduous, so you won't have screening in the winter months.

Thanks for the picture.  Regards, Marc