Two problems I see here. Let's say you plant grass that grows about 5 feet tall or a little over. What will happen when you get a good rain and wind storm is that they will all flop over and that will be the end of that. The second problem is come winter the grass will die back and you will be back to square one. Perhaps a different planting would be better, a variety of bambo maybe? I'm sure others will have their suggestions.

If you want a grass like plant, Pleioblastus viridistriatus might fit the bill - it is evergreen, but does require a little maintenance in that spent canes need to be removed periodically. In the ground, it will eventually reach about 2 metres if left without cutting down, but in a container, it will not reach anything like that height, but should reach higher than the existing planting. It looks as if your container is a good 8 inches deep - if its less than that, then it might not be deep enough. In the ground, this plant is usually cut down every couple of years to encourage fresh bright growth. If the conditions where your planter is are too shady or very exposed and windy, then this might not be the plant for you.