If you are wanting something to use as a screen all year them you perhaps should look at some of the conifers. I'm thinking of a yellow upright cone shaped type which grows quite quickly but I don't know the name, perhaps someone else can help. They are quit cheap when small. Can you put posts in with those metal spikes, taking care about drains etc.? If so you could put a trellis across and grow all sorts of climbers. I like the yellow hop because you can use it for Christmas decor when its dried.

Pieris forest flame does well in tubs,is evergreen and has beautiful flowers and foliage

Here's one ,which is about 5 feet tall in my next door neighbour's garden.

http://www.flickr.com/photos/31559373@N00/7274563836/

The foliage has faded a bit on the above photo ..the new leaves are red and turn green as they get older.... but here's a close up of the leaves and flowers in their full glory

http://www.flickr.com/photos/31559373@N00/5603711237/

if you use large pots then what about bamboo? They are tall when bought already and are evergreen too, A couple of these would make a small screen, or 3 if you have room.

Part of that depends on where you live, also. the hardiest bamboos aren't reliably evergreen. Also note that a 5'-6' bush will need a pot at least 18" tall and wide to maintain good health.

Many thanks to all suggestions. Very helpful.