I personally head down to the very south western corner of Oregon. Brookings / Harbor area is the mildest region in the PNW and the plant material that can be grown there is mind boggling. I just love to cruise up and down side streets to see what I can find. I always have a camera on hand as well. Also more than 90% of North Americas Lilies and Easter Lilies are field grown there. To give you an idea how mild the place is, I saw huge old Phoenix Canariensis, Tibouchina, Citrus, Norfolk Island pines, Large Gardenina, Huge flowering Agave americana and more. I'm going down there at the end of August. The weather is beautiful there, but can rain a ton in the Winter months. There is a nice public garden in Brookings called Azalea Park and it full of cool flowering plants and exotics.. Joe

Most important ones in northern Willamette Valley. Elk Rock Garden has British woodland garden SE of Portland, nearby Berry Botanic Garden is in same vein but less pictorial, more--well--botanical. Leach Botanical Park on E side of town also interesting. Hoyt Arboretum has tree collections. Portland Classical Chinese Garden is fun, walled and with stonework like Vancouver one but with more exuberant water features and some less familiar plants. If interested in Zonal Denial stuff like Joe was talking about Cistus nursery has plantings of these out on Sauvie Island, NE of Portland--even if you don't go for the Tropical Look particularly much of what they have is rare, sometimes their own introductions from expeditions to the wild, in short likely to excite anyone who appreciates interesting plants. (Exterior plantings could be described more as Desert or California Look, for that matter). Members of International Dendrological Society tour taken to see their private gardens (elsewhere in town) ate them up.

Look at web sites found searching with likely phrases and at guidebooks such as NORTHWEST GARDENERS' RESOURCE DIRECTORY* for more information. From descriptions contained in these you can probably tell which ones places are your cup of tea.

*2002 Sasquatch Books

Bishop's Close is a wonderful spot designed to peak in early to mid spring - it was planted in 1916 - lots of huge old magnolias, a stachyurus praecox that is stunnung, great setting on the river.
The Hardy Plant Society has garden tours during the summer - that might be something to coordinate with.

If you stop in the Portland area, don't miss the Japanese Garden in Washington Park. Beautiful any time of year and the best-designed of any Japanese garden I've seen; the spring months will bring the most bloom.

Here is a link that might be useful: Portland Japanese Garden

Thanks for the wonderful suggestions. I'll add them to my list. Sounds like there are some wonderful areas to explore and drool over!

If you are passing through Portland, you might also want to see the Rhododendron Species Garden.

Portland is the city of roses -- visit the Portland International Rose Test Garden with thousands of roses blooming in the heart of the city. First flush of blooms start in spring, April - May, reblooms all summer and through fall.

It's across the street from the Japanese Gardens --different mood and style but also very nice.

The Oregon Garden in Silverton is outstanding. It is about an hour's drive from Portland.

Here is a link that might be useful: The Oregon Garden