I'm no expert but I would advise that you remove as much root as possible. In my experience, if you don't, the shrub/plant will always come back.

I'm not an expert either, but can I ask what you're digging with?

I may have a big problem in the spring, too, as we just removed a lilac that wasn't in a good spot. In 5 years it only got 3 blooms. After having the stump ground, I kept digging, and after getting my huge hole, I took the garden fork to it, day after day for 4 days until I couldn't find any more root. I hope it worked, as I've already made a raised bed and planted appropriate shrubs.

One thing you might be interested in trying. Since you already have your shrub potted up, maybe you'd like to just bury it in the pot until spring. Then you can take it out, wait a bit, and see if the catoneaster is gone. I may wish in the spring that I'd done the same thing.

Good luck!

Well after a night's sleep I decided to tackle the hole again. I think I was too exhausted to continue last evening and just gave up. I spent some time looking for a better tool and found some kind of easy to manage pick axe. Between that and a garden fork, and my transplant spade I think I was more successful. It was really hard work being heavy wet clay. Plus I ran into some large rocks. But it is supposed to rain for the next several days and I wanted to get that shrub planted before that.
I'm stubborn in that I hate having to wait for DH to help me with these projects. He has no interest in my garden.
Marric, your advise encouraged me to try harder. NAF, I'd already finished before seeing your post. Thank you both for responding.

Please let us know in the spring whether the darn catoneaster comes back or not.