Knockout roses are grown on their own roots, not grafted. You should see healthy, light colored roots which are not bound up in the pot.

Do Knock Out roses have big roots?

How Deep Does a Knock Out Rose Bush Grow? Knock Out rose root depth varies due to soil density and plant age. Knock Out roses begin in an 8- to 10-inch-deep hole with a spread of at least 24 inches.

Can you make a Knock Out rose bush into a tree?

Rose shrubs can easily be pruned and trained into a topiary tree form when young. Prune away all of the canes and branches that emerge from the central leader on the bottom 2/3 of the plant.