OOps, correction:’details are not clear to be certain’, so sorry! If this is loosestripe, you might find this link interesting for Lythrum salicaria species. If it isn’t that, make new post with closeup of flowers and stems & leaves. http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=b513

Details are clear to be certain. I suggest you check out cultivar of Lythrum (loosestrife) and see if you have a match. Check out these included links for further information. Best wishes! Link for one of its cultivar L. salicaria ‘Robin’. https://www.dobbies.com/products/plants/cottage-garden-plants/lythrum-salicaria-robin/