If Knock Out roses aren't pruned, they don't produce as many flowers or grow as well, according to Gardening Know How. Nonetheless, the NC Cooperative Extension recommends that Knock Out roses shouldn't be pruned for the first or second season anyway, as it gives the bush a chance to mature.
Most gardeners recommend that Knock Out roses be pruned once a year in late winter or early spring, according to the University of Maryland Extension. Knock Out roses should not be pruned down to less than 2 feet, notes the LSU AgCenter, and gardeners should prune them to anywhere between 1 foot and 2 feet below the eventual height they want the plant to be, as the plant grows in abundance each season, notes both the LSU AgCenter and the NC Cooperative Extension.
The LSU AgCenter also recommends a second pruning in late summer, about the end of August. However, this pruning should be lighter than the spring pruning, only reducing the plant by about one-third of its height. On the other hand, the NC Cooperative Extension notes that some experts say that this type of heavy pruning need only be done every couple of years to encourage growth, rather than every year. Also, light pruning can be performed throughout the season to help make the shape of the plant more aesthetically pleasing, according to the NC Cooperative Extension.