Do you know what his wild perennial is?
Zone is 8a, central Europe. Photo taken mid-May. Removed from the ground because of fence construction. Is it worth transplanting?
Google images thinks one of your photos looks like "large-flowered evening primrose".
I'm making my comment an answer - Symphyotrichum novae-angliae (syn. Aster novae-angliae). We really needed to see the flowers on this one, though.