You would have more luck getting them started inside. You would not need a big pot just something big enough for them to fit in until they start growing.

The warmer they are the faster they will start to grow. Our houses are usually warmer that 50-60 degrees.

You want the soil to be damp, to get them started. In the summer most of my cannas if grown in full sun can be grown in bog condition which is soupy wet conditions.

If you can get them growing good before May you can get flowers as early as July. This bloom time also depends on which canna you have. Some of mine bloom faster than others. Because our summers are so short I buy the cannas with colorful leaves and do not worry about the flowers. Some of the flowers really are not that spetacular or pretty anyway.