You could look at a Bobo, even try it in a container in front of the knockout

Little Lime might work. It will get taller than Bobo - at least mine do. Little Quickfire might be too wide.

Thanks for the suggestions, I would prefer the Knockout in front. Little Quickfire will probably clash with the soft pink of the Blushing Knockouts.

Knockouts get to 6' after a few really good seasons without deep freezes. Plus side is you can prune to size/shape.

Little Lime can also achieve 6'+

springwood- my knockouts never get that big here. We get some pretty cold winter weather and late spring so they never get more than 3-4 in height.