I just spent a week winding through the weird back roads of Wisconsin and this plant, Hesperis matronalis, ruled the roadsides. You can call her Dame's Rocket. She screams at you to notice her in ditches and roadsides in bitchin' shades of neon violet. Invasive? Maybe. Depends on where you live so none of your sass.
1. Smells great
2. Butterflies and bees love it
3. I can't get over the color/height for this time of year
So, yes, it's a crazy reseeder. So be the boss, deadhead or pull the whole plant, or stand back with your hands up. I sprinkled 30000000 Hesperis seeds on the borders of this property when we moved in and not one plant came up. I think this plant, like so many sloppy reseeders I have known, likes to have some say in the matter. Much like a toddler. So start one plant from seed, plant that and then you'll have a purple army.
Is it a Phlox? No. It's in the cabbage family. Weird right? Totally not a Phlox.
Is it Lunaria annua? No, but gosh, they look a lot a like. The big differnce in my book is the "money", obvi. Also a crazy reseeder and blooming RIGHT NOW. I wonder if they could be friends?