Now would be a great time! I am in the Rogue Valley and we might have to be a bit careful yet with the more tender bulbs such as dahlias, but Portland should be fine right now. You guys have a much milder "low" temp for rest the season if I remember right (I have lived in Vancouver, Salem and Brooks). To be a million percent positive, call a local nursery, NOT a big box store, but an actual nursery. That area of Oregon is alive with them and all would be more than willing to help. Happy planting of your dahlias! Aren't they a superb flower with seemingly limitless varietys and colors?