I love the Rose Test Garden and when I was looking for outdoor venues, the Shakespeare Garden immediately sprung to mind. It is in a quiet corner of the rose test garden and has a built in "alter" for the ceremony to take place. No additional...
decorations are required. The roses are not very prevalent in the Shakespeare Garden itself but they are everywhere on the grounds. The classic views of Mt. Hood, the water displays, the sculptures and all of the color everywhere make for gorgeous wedding photos.
Be aware that when trying to reserve the gardens, you are only allowed to reserve it for 2 hours and everything must be cleared after your time slot. Also, no weddings can be held in the garden during the rose festival. When you go to reserve the garden, you can only do so during the calendar year of the event and the gardens can fill up fast. Your best bet is to go (in person) to the park and recreation office on the first business day in January of the year you wish you hold your event. The line was 65 people when my best man went at 8am on January 3rd, 2011.
Also, of note (and one of those love hate things about my wedding), you cannot control who pokes their head into the venue or who stands above the Shakespeare Garden trying to get a peek of the wedding. I will have several interesting photos of total strangers standing over our ceremony. Oh well... I'll deal with that because of how wonderful all of the people in the gardens were to me before the ceremony. Everyone loves a wedding and the congratulations and well wishes will brighten any day. I was a blushing bride before I made it to my starting place for my processional. It's a very public venue so just go with the flow and if it really bothers you, place someone at the top of the garden to shoo people away from your pictures. Not bad for only about $350 for a venue.