I had never heard of Shirley poppies, but I've now Googled it to see if they are the same as my Oriental poppies. They appear to be the same plant but with different shades of colours mostly. So, I would say, Yes, if you deadhead, the should continue to bloom. My Oriental poppies will, but only if I remove the stocks of the dead ones.
Some of the images that came up for Shirley poppies looked beautiful! I'm going to have to watch the seed stands in the stores and watch for Shirley poppy seeds. I'd like to try them.
