Pacific Northwest Gardens
I love touring gardens. My favorite is Kew Gardens. A couple of years ago I stayed at Crathes Castle in Scotland, which holds an incredible garden (the yews were planted in the 1700's). So, yes, I know that the gardens I'm going to list here are not the fanciest in the world, but they're still very enjoyable and worth a visit if you happen to be in the area.
The first arboretum I grew to know and love spans 200 acres next to the University of Washington. Compare that to the 50-acre Butchart Gardens which is wonderful (if costly and Disneyfied) and my all-time favorite, Kew Gardens, which encompasses 300 acres. Washington Park Arboretum has a nice Japanese Garden and loads of trees and woody shrubs, mostly of Pacific coast and Asian origins.
Washington Park Arboretum
The best Japanese Garden I've ever seen is the Japanese Garden in Portland, only 5 1/2 acres, but loaded with charm.
Japanese Garden of Portland
Another tiny gem is the Weyerhauser Company's Bonsai Collection -- just an acre in size, but there are a couple of mini-forests there.
Pacific Rim Bonsai Collection
The Pacific Rim Bonsai Collection is adjacent to the Rhododendron Species Foundation which covers 22 acres and is home to 10,000 Rhododendrons. Ho-hum, I don't like Rhodies, I'm just putting this in because so many people do.
Rhododendron Species Foundation
The 150-acre Bloedel Reserve on Bainbridge Island is one of my favorite gardens. It is known for its magnificent prospect looking north across Puget Sound to Mt. Baker and the Northern Cascades as well as its huge natural stands of native Douglas Fir, Western Red Cedar and Western Hemlock. (Bloedel was a lumber baron.) Its unbelievable moss garden, 6 acres across, is said to be the largest anywhere. Tours are by reservation only.
Finally, Lakewold Gardens, ten acres in size, is another private garden that has re-opened for tours. An interesting bit of trivia about Lakewold -- its original owners were related to the owners of the Bloedel Reserve -- the wives were sisters. Lakewold's claim to fame is that it is home to the fabled Himalayan Blue Poppy, a strange plant with an incredibly sky blue blossom. My favorite at Lakewold are its magnificent copper beeches.