When do you cut back knock out roses??

late winter when they are dormant

I cut them way back before the first frost, but if you missed that, they cut them back as soon as the first frost is over in late winter. They should thrive.

As of right now...early spring

Once your Knock Out has reached its mature size of 3 to 4 feet tall, usually in its second or third season in your garden, it is ready for its first spring pruning. When you see buds begin to swell to send shoots out of the canes in late winter or early spring, you can cut each cane down as low as 12 inches and expect it to grow back to that mature height by the time it blooms five weeks later. Prune as you would a traditional rose, cutting 1/4 inch above a bud at a 45-degree angle.