Very tall annual or biennial flower

Lyn

If you’re in a warmish sheltered area you can get 20’ + out of an Echium Pininana 

Blue Onion

A second for hollyhock.  Also cleome.  

CharlotteF

I had an ammi majus get to 6-7ft one year. I don't think they're meant to do that! But as Fire says, rich soil grows giant plants sometimes!