I don't know Sunset Boulevard but when you said Oenothera..my immediate reaction is THUG . Hopefully your variety is different than the standards that create such a mat of runners and roots so bad I had to toss both the ground and the roots/plants to get rid of it.
Off to Google for your variety.

But, some of the more upright varieties behave very nicely, such as evening primrose... and I'm not familiar with the one you have. It's the short spreading varieties (usually pink or white) that you really have to watch out for. Maybe if you can't get good variety info- start it in a big pot and see if it spreads to fill the pot fast (or comes out the holes in the bottom- a sure sign you have a spreader.)

Should have mentioned even the thugs are really beautiful. So it's worth growing in a pot and deadheading if it's a spreader.

All I can say is that I rue the day I planted the typical pink-flowering variety. It is in my beds and it spread to my lawn...if I could only turn back time and undo this.

Thanks for the replies! I will pot it up and see what happens. I planted some anenomes (sp?) once and it took an entire year to get rid of them all. Did the same with one or two other thugs, so I like the pot idea! ;-)

Sally