Betty our short summers are kind of hard to get Cannas tall early in the season. They like hot, hot, hot and wet, wet, wet. So they do not take off until it gets really hot. If you want privacy I might would try some type of vine planted in pots with trellis in the pots. Honeysuckle is fast and hardy and for color in the pots you could plant coleus and some summer flowering annuals.
I am only saying from my experience because we have such a short period of time of growth these cannas do not grow fast. If you start them this year and do what I do keeping them half alive all winter you will get a tall plant sooner but probably not as tall as you want/need early in the season.
