In the spring, when the Forsythia are in full bloom, is a safe time to prune your knock out roses.
Here are 3 different methods.
- The first method is to not prune them at all. If you don't prune them, knock out roses (depending on the cultivar) will grow up to 5' tall and about 4'-5' wide. If you follow this option, it might be a good idea to thin them a little every few years to keep them looking nice and blooming well.
- The second method is to cut them back hard and thin out some of the oldest wood every few years in the spring after the threat of cold weather. You can prune them back to 18", 24", 26", or even 48" from the ground.
- The third method is to cut them back every year in the spring after the threat of cold weather. You can cut them back hard as above or you can just cut them back enough to shape them they way you want them to look. Thin them out, too, if necessary to improve their appearance and blooming.