"Leach Botanical Garden is located in a woodland hollow carved by Johnson Creek in southeast Portland." Check out it out--free!
"The Garden’s collection of more than 2,000 different species and cultivars can be enjoyed on a self-guided tour that winds around the Manor House and along Johnson Creek, past stately trees, native and exotic shrubs, extensive displays of ferns, woodland lilies, and alpine plants."
Leach is a wonderful setting to host events, too. Learn more: http://www.leachgarden.org/