Knock Out roses should be pruned in late winter or early spring, according to the University of Maryland Extension. The Louisiana State University Ag Center also recommends a light pruning in late August or at the beginning of September.
Knock Out roses do not need extensive pruning, according to the University of Maryland Extension. They can either be pruned to about 2 feet or they can be pruned to about a foot shorter than the gardener's ultimate desired height for the bush. Pruned height should be no less than 2 feet, advises the LSU Ag Center. The pruning in late summer should be light, leaving at least two-thirds of the plant.