Swansons has both a great variety and high quality of plants -- I've always been pleased with anything I've purchased there. Things are arranged in an easy to understand way, but evergreen, perennial, annual, etc. They also have a great staff that is happy to help you pick the right plant or tree for your location.
The only drawback is the higher prices, compared to places like Home Depot or Fred Meyer. I tend to shop those places when looking for something ordinary like pansies or mums, and save Swansons for when I need something more unusual or interesting.
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