Q. Can you suggest a dark green evergreen focal plant for large concrete planter with planting area measuring 17" X 52", approx 3 feet high.
Your biggist limitation is the 17" wide container as some freezing occurs in the winter. You could try a hedge of Ilex crenata, Japanese Holly `Sky Pencil' which you could keep trimmed to 3 or4'. It is good to have a buffer between the roots and the concrete (I.e. bubble wrap or similar to prevent roots from freezing). Anotoher alternative would require lattice or a fence to attach a plant (some varieties of upright yew or juniper) that could be espaliered on it. If wind is an issue lattice would be the better choice. The Japanese holly would have less aggressive roots than either of the other two shrubs.