Hi April,
I'm not sure about Eugene specifically, but I believe you would have to keep them there just like I do here up north a little bit. At the end of October everything gets their leaves stripped and they are brought into the garage, where I make sure it doesn't freeze all winter. Then in April around last frost they go back out.
I usually keep over 50 brugmansia through the winter (slightly obsessed). Definately bring 'em!
- Tom

I understand the "slightly obsessed", I have 30+ cuttings rooting now. Looks like I'll be bringing them! Thanks!

later...April

You're moving into Sunset Zone 6.

ZONE 5. Ocean-influenced Northwest Coast and Puget Sound
Growing season: mid-April to Nov., typically with cool temperatures throughout. Less rain falls here than in Zone 4; winter lows range from 28 degrees to 1 degree F/-2 degrees to -17 degrees C. This "English garden" climate is ideal for rhododendrons and many rock garden plants.
ZONE 6. Oregon's Willamette Valley
Growing season: mid-Mar. to mid-Nov., with somewhat warmer temperatures than in Zone 5. Ocean influence keeps winter lows about the same as in Zone 5. Climate suits all but tender plants and those needing hot or dry summers.

Here is a link that might be useful: Sunset's Garden Climate Zones 1-15