Question
I live in the Philadelphia, Pa area.
I would like to plant some plants on top of a 2 foot high retaing wall.
The soil on top of the wall is about a foot deep with stone below.
I would like  some plants that will live year round so i don't have to put plants in ever year.
What would you recommend that won't grow to big ?
I would like them to be no bigger than 1 foot in diamater.
If possible is there a plant that would also flower ?
Can i plant them now or do i have to wait for the Spring to plant them ?

Answer
You can probably install some nice looking plants right up to Thanksgiving if the weather holds out.  So long as they are heading toward dormancy, you won't have any problems - they'll look awful, so expect that.  But they'll be back in the spring, rested and good as new.  Autumn is the preferred schedule for transplanting because the metabolism of the plants is headed down, not up.  In spring, it's the other way around.

Now for some suggestions...

I'm partial to spring Phlox subulata when it comes to walls and spring.  Have a look at the colors at the Bluestone Perennials website:

http://www.bluestoneperennials.com/

The Phlox will stay fairly compact, trailing just a little over the wall as it matures.

For a more gregarious effect, consider the look of Sweet Autumn Clematis, C. paniculata.  This will cover the whole top and any supports you provide across the wall --- you'll have to watch it and see how it blooms.  This is generally something that goes on a fence because of the support, but you can give it a shot.  The foliage will totally cover everything.  Phlox blooms in spring; Clematis is a late Summer bloomer, and both are very fragrant -- and will bloom cooperatively in semishade.

Speaking of fragrance and semishade, Honeysuckle -- Lonicera halliana -- may not be the prettiest thing you'll ever see, but makes up in scent what it lacks in show.  This is essentially a weed, but not terribly invasive and easy to keep under control.  The fragrance is a 2 week trip into heaven, too short for me.

Just a note: If you are contemplating a different Clematis, keep in mind you will need more sun for other Clematis.  You have not mentioned the sun exposure, so I'm a little conservative here.

If you do have a lot of Sun shining down on that wall, I heartily recommend one of the Sun-loving Clematis.  They may not have a fragrance, but they are bright, cheerful and bloom like crazy in all sorts of vivid colors.

All the above are perennials, which is what you really should do.

There are some annuals, but as you know they will have to be re-planted year after year.  If you want to look at those, let me know and I'll assemble a list.  I will need to know how much sun you can give them, please remember to include that.