can the be successfully planted in a container?
It would have to be a large container (with drainage holes) for a Knock-Out rose to grow and thrive. I'm thinking 24" across and 18" deep. If you live where winters are cold, protection of above ground roots from freeze and thaw cycles is vital. Use potting mix, not garden soil and a long-acting fertilizer. Another concern is being blown over in storms. Consider adding a brick or good-sized stone in the bottom of the container. A better choice for growing in pots is the landscape rose. These only get 2 ft high, bloom prolifically and are available in many colors. One brand is OSO Easy.