To maintain a size of 3–4’ w x 3–4’ h, Knock Out Roses should be cut back once a year to 12” high. Check your rose bush from time to time in late winter/early spring, and when you start to see new shoots growing from the canes on your rose bush, that’s a good sign that it’s time to prune.
Jan 08, Some people prefer to do two pruning every year - first time in January when you should cut back by about half their height. The second time you should cut back at the end of August but not very severely. This time cut them back by only about one-third of the shrublop.buzzted Reading Time: 4 mins. Mar 01, Depending on where you live, you can technically cut your knock outs back anytime, but I recommend doing a drastic prune in late winter/early spring. Have you pruned them lately? For the health of the plant you should prune it back by 1/3 to 1/shrublop.buzzted Reading Time: 4 mins.
Jan 13, The degree to which you are pruning would be considered rather severe for Knock Out roses. The heights you are using apply more to hybrid tea and grandiflora roses. Research indicates that Knock Out roses respond best if not cut back lower that 24 inches for the hard pruning in early spring, and more moderately in the shrublop.buzzted Reading Time: 6 mins.
Grumpy: Even a 'Knock Out' looks woozy after months of Southern heat. So put on some leather gloves to protect you from its vicious thorns and then use hand pruners to cut the plants back by about one-third.
Next, fertilize them according to label directions with Miracle-Gro Water Soluble Rose Plant Food. So, you want them to be about 3' tall? I'll tell you that Knock Out grows over 5' tall here, and that is with a hard (down to 18 inches tall) pruning. If you want to keep them at that height, you'll have to cut them down during the growing season. They rebloom quickly, so that shouldn't be a problem.