Extremely invasive, and spreads everywhere you Don't want it!!
"eyes"
Oenothera speciosa
Have you added your comment on it to the plant's entry in Plant Files? That's the best place to warn people about its bad behavior, then anytime someone looks it up in Plant Files they can see how many positive and negative comments people have put in about a given plant and then decide whether or not they really want to plant (or keep) it.
It's already there.
http://davesgarden.com/guides/pf/search.php?q=+Oenothera+speciosa&Search=Search+PlantFiles
"eyes"
I know it's already there, what I meant was add a comment to it--if you go to the bottom of the plant listing, you'll see a place where it'll say how many positive/neutral/negative ratings there are and then you can read people's individual comments on it and add your own. I didn't see a comment from you among the ones that were there, that's why I was suggesting you might add it there. If someone has this plant (or is thinking of buying it) they're much more likely to look at the PF entry and read the comments there than they are to do a forum search and find out that you said here that it was invasive, so you'll be helping more people avoid the plant if you put the comment in Plant Files. There are already other negatives there, but the more the merrier! The more negatives there are, the less likely someone is to consider planting it.