Why are my knockout roses, which are several years old, sprouting wild rose branches? I have pruned these out, but they reappear.
It sounds like your Knock Out rosebushes are the grafted type of rosebushes. The hardy rootstock they are grafted onto will sometimes decide to send up its own shoots/canes. It is best to do as you have been doing and prune them out. Be sure to prune them out as far down on the bush as possible, even a bit below ground and seal the cut with Tacky Glue from the craft store. If these shoots are not pruned out deeply they will come right back very quickly. Please see this article link on Own Root and Grafted rosebushes too: https://www.gardeningknowhow.com/ornamental/flowers/roses/own-root-roses-grafted-roses.htm