Last spring, I started redoing the landscaping at the office. There were several tall white phlox on the south side. I accidently pulled up a piece that was about six inches tall. I planted it on the east side of the building and forgot about it. It is now about a foot tall and has this huge blossom on it. Actually the blossom is larger than the ones on the 3 ft tall plants on the south side of the building.
I have them in several colors at home - pink, white, red, purple etc. I think I posted pics awhile ago of them.
I wonder if the 3' ones on the south side of the building would benefit from a 'haircut' and then, maybe, just maybe, they'd flower as well as the one in your photograph.