Well I'm just completely smitten with Coleus this year so that would be my suggestion. It's a plant grown for foliage and there are so many colours to choose from. Your light conditions are right and they are suited to growing in a container - they're also suited to growing as houseplants and cuttings will root in a glass of water (so I've read - I haven't tried this yet). The only downfall I can think of here is if you get a lot of wind on your deck - I don't know how well Coleus would handle a lot of wind.

I can't think of an annual grass that would be happy in shady conditions. Maybe Dracaena spikes? You know, those spiked plants that are always in the center of big containers. You can pick up many right now at Wal Mart and pot into a larger container for a couple of months and keep them in a high light area in your house. (Disposable beer cups with a hole punched in the bottom for drainage are a very good alternative for those who may not have a bunch of pots hanging around). They'd be a nice size by May/June when you'd be potting into the long planters and you'd have the instant height you'd need.

Good luck!

I was stumped with this question, but I like Pudge's idea about the Dracaena spikes.
I grow one on the east side of my house, gets morning sun and thats it and it grows quite well.

Sharon

Coleus would be a good idea, and I have only had success rooting it in water. As a rule, it shows root nubs on the 7th day it is in water, but I just snipped a tip off of my Peter's Wonder to see how long it would be, and a root jutted out in 2 days. My Peter's Wonder I overwintered grew more than 20 inches in height, and lots of side branches, so I will have lots of plants where I need them.

The Cordyline spikes would be good to, and I've read the redder hued ones need less sun, actually they require some shade, to do well. And they are at the height you're asking about also. Depending what shades of coleus you go for (if you do) they could be nicely matched with the cordylines.

Coleus roots very well in soil. Take your cuttimg just below a leaf (the roots come from a leaf node) Remove all but the top 3 or 4 leaves. Stick the cut end into rooting hormone--it comes in powder or gel--I use stim root No. 1.
poke a hole in the pottimg soil in your pot and drop the in cutting and cover it in. In a week or two you will start to see new growth and that means you cutting is rooted but wait a few weeks for it to be rooted good before doing anything with it.